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Spoilers for all Metroid entries released prior to this one, including Super Metroid, will be left unmarked. You Have Been Warned!

"I am already detecting massive bio-signs in this region. The X are gathering. This may be our chance to exterminate them, but... You are only at 10% battle capacity. Your chance of survival is extremely low. The Federation needs you alive and on duty, Samus..."

Metroid Fusion is an action-adventure platformer video game published by Nintendo for the Game Boy Advance. It was released in North America, Europe, and Australia in November 2002, and in Japan in February 2003. The game is considered to be the fourth main installment in the Metroid series; the intro alternatively refers to the game as Metroid 4. Metroid Fusion was developed by the same development team that created the previous Super Metroid, to which Metroid Fusion bears heavy resemblance.

Chronologically, Metroid Fusion takes place second-to-last in the fictional Metroid universe. The story centers around bounty hunter Samus Aran, who is sent by the Galactic Federation to assist Biologic Space Laboratories in their expeditions on the planet SR388, which Samus had purged of Metroids in Metroid II: Return of Samus/Metroid: Samus Returns. While defending the scientists from the local wildlife, she gets infected by a previously undocumented gelatinous lifeform and later passes out while piloting her gunship. The organism has spread throughout her body and her biomechanical Power Suit, and the scientists surgically remove portions of the infected suit to no avail. In a last ditch effort, the scientists infuse Samus with a serum containing preserved Metroid DNA, which destroys the parasite and cures Samus.

Now sporting much altered armor, Samus is sent to the B.S.L space station to investigate an explosion. There, she learns that the station is swarming with organisms infected by more of the gelatinous lifeforms — now given the name X Parasites — organisms that can infect a host body and take control of its central nervous system, allowing the X to control the host body. The X Parasites are then able to become an exact copy of that host, complete with the memories of the infectee. The fact that Samus now has Metroid DNA within her allows her to absorb other X Parasites. This is because the Metroids, genetically engineered creations of the Chozo, were made to be predators of the X; the recent Metroid purge let them infect the creatures on SR388 and spread once more. In order to check the spread of the infestation, Samus must work her way through the station under the guidance of her new gunship's Artificial Intelligence and eradicate the X Parasites and their host bodies.... But among them is the SA-X, a clone produced from the remains of her Power Suit, with all of her weapons and powers at its disposal.

The game is notable for having a much more in-depth plot than the previous games in the series, full of political and corporate intrigue and character interaction. While the first Metroid Prime game, released the same year, opted to tell its story primarily through scan logs, Fusion tells its story through dialogue with an AI commander and through Samus' Internal Monologue. As a result of this heavier emphasis on story, the game is more linear than the previous games as well.

The game can be hooked up to the first Metroid Prime to unlock both Metroid and the Fusion Suit skin for Prime; those with Metroid Prime Trilogy for the Wii can unlock the Fusion Suit through its bonus credit system.

An eight-page Comic-Book Adaptation titled Metroid Fusion Special Edition: Rebirth of Samus was produced by the artist of Metroid: Samus and Joey as bonus content for its first volume; it covers the events of the game up to the first Boss Battle in abbreviated form.

The game got re-released on both the Nintendo 3DS (as a part of the console's Ambassador's Program in 2011) and the Wii U's Virtual Console lineup in 2014.

Nearly two decades of Development Hell later, the long awaited follow up, Metroid Dread, was announced for Nintendo Switch. A few years after Dread's release, Fusion would be added as part of the Nintendo Switch Online Expansion Pack service in March 2023.

This game contains examples of...

  • 11th-Hour Superpower: The Screw Attack is obtained extremely late and is useful for connecting all the sectors up, but the main example is the Ice Beam, normally an early-on or midgame upgrade, which is gained from the SA-X along with an unnamed suit when the Omega Metroid kills it at the end of the game. This allows Samus to handily win when it was hopeless before.
  • Abandoned Laboratory: The game takes place in a space station made of these.
  • Action Girl: Samus, at her most prominent. Even after facing a deadly parasite and given a minimal chance of survival, she's still up for taking bounties, as well as her deadliest mission yet.
  • After Boss Recovery: All the bosses and enemies are made of X Parasites that Samus absorbs to recover health, and the bigger bosses have more potent X for her to eat.
  • A.I. Is a Crapshoot: Ultimately subverted with Adam. At about the point one might expect him to start singing "Daisy Bell," he sides with Samus against the Federation and helps her destroy the X Parasites. It ultimately turns out that Adam's not a true AI either; he was human to begin with, and just had his mind uploaded before he died.
  • Always Chaotic Evil: All X Parasites are extremely dangerous lifeforms that kill indiscriminately but also have enough intelligence to not count as wild animals.
  • Armor-Piercing Attack: The Plasma Beam pierces through tough enemy armor in addition to being the strongest beam. The armor piercing capabilities of it are potent enough that it finally allows Samus to challenge the SA-X on equal footing. Which is exactly why the Federation had no plans to give it to Samus, as they hoped to study the X and SA-X, believing them to have "endless potential applications." Samus ultimately has to stumble upon it herself.
  • Artificial Brilliance
    • The X in general. The first few X will attempt to be absorbed, because the X are trying to infect Samus. After a while, when they realize she's just absorbing them, they'll try not to be absorbed. And then when the Cold-X start appearing, the cycle repeats.
    • The SA-X as well, utilizing a bit of a Call-Back, will attempt to freeze Samus with the Ice Beam, and then blow her to bits with a Super Missile because that's the most efficient way of killing a Metroid. If Samus is hanging from a ledge, the SA-X will fire a Super Missile at it to make her lose her grip.
  • Artificial Stupidity: The SA-X also has absolutely horrid looping programming, to the point most can be exploited for humor. Power Bombs in particular seem to completely throw its programming into an infinite loop when Samus isn't in a room needing the Power Bomb to advance. This can be exploited in the boss fight with the SA-X. There is a ramp above the recharge station in the room where you must send the space station into SR388. Continuously charging and shooting SA-X will cause her AI to instinctively charge at you with a Screw Attack in an infinite loop as long as you know what to do. Some of this is intended.
    Samus: (internally) The SA-X. An enemy with my strongest abilities. But does it have reason? Probably not. It's just a killing machine. In my current state, I can't face it head-on. The SA-X is me, only heartless. I must stop it before it learns its potential and threatens the universe...
  • Asteroid Thicket: During the opening, Samus' ship crashes into one of those. The asteroids look like they're only a few hundred feet across, and tightly packed.
  • Background Music Override: After Adam orders Samus to propel the B.S.L station into SR388, the song "Last Instructions" plays everywhere until Samus starts the propulsion sequence, when it's replaced by "Station Escape Timer Mission."
  • Bag of Spilling: Played with. The opening cutscene shows Samus in possession of all her upgrades. Unfortunately, she and her Power Suit become infected by the X Parasites and large portions of the suit are surgically removed in an attempt to stop the spread. Samus begins gameplay with none of her abilities beyond the Power Beam as a result of this, but the upgrades do still exist, just in the form of an enemy who's all too willing to use them on her.
  • Bait-and-Switch Boss: A strange case of a boss being a bait and switch for itself. When you first get to the water-themed Sector 4, Adam mentions it was used to host the powerful Serris and that the X freed it for unknown reasons, suggesting a fight with the beast. And then you find its skeleton, a few rooms before having to fight an X clone of it. Something similar happens much later in the game: the frozen body of Ridley crumbles to dust when you touch it, as the X already infected and copied it.
  • Beauty Is Never Tarnished: Parts of Samus' suit have to be surgically removed, but not all of it could be taken off. This is due to much of her suit "fusing" with her central nervous system. She is given Metroid DNA to counteract the X Parasite. This, along with her Chozo DNA, does not seem to affect her appearance in her death animation, though. It is also unclear whether or not she can ever remove her suit by choice.
  • Being Good Sucks: At the end of the game, Samus sets the B.S.L station on a course to crash into SR388 in order to kill all of the X Parasites in both locations, going against the wishes of the Federation members who wished to study the SA-X. She fully expects the public to hold her accountable and launch investigations for doing this due to not knowing the full reasoning behind her actions, but Samus fully believes that she made the right choice.
  • Big Bad: The SA-X. It was the one responsible for the disaster and for releasing all of the captured X into the station. It is also responsible for all of the tactical and deliberate damage done to the station that hindered Samus's progress and blocked off necessary areas.
  • Big Eater: Samus's new Metroid DNA allows her to devour X Parasites in large numbers to heal herself, as opposed to the series norm of unspecified "Energy" drops.
  • Bittersweet Ending: Samus saves the galaxy again and reunites with former allies (even one previously dead). But in the process, she permanently destroys the B.S.L space station and SR388, and was forced to defy orders to do so. Samus got new friends, but is now likely to face scrutiny for seemingly committing an act of terrorism against the Federation. However, the Japanese script shows that the Federation isn't politically monolothic and is made up of multiple of branches (something that was Lost in Translation in the English localization), and the beginning of Metroid Dread shows that she's still on good terms with the Federation. So, if someone within the conspiracy party tried to brand Samus a terrorist, it was an accusation that didn't hold in court.
  • "Blind Idiot" Translation: Prima Games's two-in-one Strategy Guide for Metroid Prime and Metroid Fusion calls the Fake Chozo Statue "Elephant Bird", a mistranslation of "Birdman Statue", the Japanese name of Chozo Statues, since "Zō" can also be translated to the word "elephant" with a similar kanji.
  • Blue Means Cold:
    • The general color of the cold-themed Sector 5, ARC, is blue. The lights for its elevator are a bright cyan, the B.S.L tiling in the non-cold rooms are various shades of pale blue, and the cold rooms are vibrantly cyan and blue.
    • The light blue X Parasites freeze Samus solid for a few seconds.
  • Body Horror:
    • The X provide excellent examples of this trope; it is in their very nature to absorb a lifeform from within and either impersonate them for a specific task, or modify the body of their host by using the genomes of previous hosts. This has interesting and disturbing effects, such as turning humans into zombies made of regenerating goo. They also turn Hornoads (an enemy from Metroid II) from a relatively docile creature into a slavering, aggressive monster with More Teeth than the Osmond Family.
    • For a prime example, see the last battle with the SA-X. In desperation, it transforms from an exact Samus replica into what looks like a mashup of multiple creatures, whilst still showing bits and pieces of the original Power Suit embedded in its flesh. Notably, it resembles most a Hornoad, which was the last host that particular X had before infecting Samus.
  • Book Ends: The game provides four. The end to the original trilogy had the Baby Metroid heal and give you a weapon to defeat the final boss, saving your life. In this game, the Metroid's DNA cures Samus of the X Parasite infection (saving her life), and makes her able to absorb the X to heal her and replenish her supplies (giving her the weapon to eradicate the X). Also the ending, in which the SA-X, (inadvertently) saves your life, heals you, and gives you the weapon to eradicate the Omega Metroid. Also, both the very first boss (Arachnus) and very last boss (Omega Metroid) are from Metroid II. Lastly, while this is likely unintended given that this game came first, Neo-Ridley's screams are the same as his original counterpart's screams in Metroid: Zero Mission, which is a remake of the first Metroid game - timeline-wise, Zero Mission was his first appearance in the series, Fusion was his last.
  • Boss Tease
    • Subverted with the SA-X. You are clearly told to run like the Dickens whenever it appears, as it is strong enough to kill you within seconds and, even when you get a weapon that can affect it, you cannot deal it any damage; merely slow it down. However, in the end, you'll get the battle you truly wanted.
    • Nightmare appears in the background when Samus leaves Sector 5 for the first time. It flies around in the background as you progress, causing rooms to shake violently. However, you cannot fight against it immediately - it is only after an explosion occurs in Sector 5 and Samus is told to investigate the ruined sector when you can finally revisit the area and start the actual boss encounter there.
  • Brain Uploading: The AI aboard the ship given to Samus by the Federation is revealed to be Adam Malkovich, Samus' former CO. It seems the Federation regularly uploads the minds of important military, political and scientific figures.
  • Breaking the Fourth Wall:
    • A hidden Easter Egg can be found if you abuse the Shinespark to get Samus to a Navigation Room after defeating Nightmare but before getting the Diffusion Missiles. Doing so will cause the AI and a mysterious figure to congratulate her. The secret message ends with the former musing: "I wonder how many players will see this message?"
    • A particularly terrifying example occurs with the SA-X's introduction. Where upon breaking into a room Samus just left via elevator, it stares DIRECTLY at the player. Complete with a close up of its face.
    • For beating the Japan-exclusive Hard Mode with 1% of items collected, you get an ending showing a shelf full of action figures of Samus, the SA-X, the Baby Metroid, and a Missile. The brand these toys belong to is... Metroid Fusion, complete with the game's own logo on the packaging.
  • Breather Episode: The mission to investigate life signs on the Habitation Deck: It comes right after the intense Timed Mission to prevent the Main Boiler from exploding, the navigation is simple, there are relatively few enemies, and there's no Boss Battle.
  • Call-Back:
    • In the lower part of Sector 4 after you beat Nightmare, you have to Power Bomb a certain glass tube to move forward, just like how you accessed Maridia in Super Metroid.
  • Cartoon Creature: Nightmare is a bizarre bio-mechanical lifeform that doesn't land in any identifiable species.
  • Central Theme: Sometimes to be a true hero, we have to become what we fight against. This is seen two-fold throughout the game:
    • On her own, Samus is ultimately powerless against the X. Her attempts to ward it off on SR388 amounted to nothing. When she was injected with the Metroid Vaccine, she became partial Metroid herself, and thus completely capable of destroying the X. She became the very thing she's been fighting in just about every game to date.
    • Despite being a lone-wolf bounty hunter, Samus clearly still cares about maintaining stability in the galactic civilization. However, she recognizes that the Federation is WAY out of their league when they decide to capture the X, especially the SA-X, for their military applications. However, the only way for Samus to solve the issue and prevent the X from leaving is to crash the B.S.L station into SR388 and destroy the X that way. However, in doing so, her action is basically interpreted as an attack on the Federation and thus the galaxy at large, the very thing she's constantly fighting against. However, Adam reassures her that somewhere, someday, someone will understand.
  • Checkpoint Starvation: Once the power goes out, the only place you can save is your ship. Your next objective is a long slog through the core against powerful Kihunters, fully-powered Zebesian Space Pirates, and the Yakuza boss, all of which are a huge step up from the enemies you've been fighting so far. Once auxiliary power is restored, a save point is right next door. It's even worse in 1% completion, because most of the enemies can kill Samus in one hit, including the boss, which is one of the contenders for the hardest boss in the game.
  • Chekhov's Gunman: The Etecoons and Dachoras from Super Metroid come back in this game, and this time THEY save you. After needing saving a second time, but still.
  • Chest Monster
    • One of the Core-X impersonates a Chozo Statue holding a powerup. Touching it damages you and prompts it to revert to its true form for a boss fight. The fact that it is revealed to be holding the already-acquired Morph Ball gives it away.
    • There are also traditional chest monsters. There is one hostile Energy Tank and a Missile Tank mimic in the game with the real one sitting in the same room or the one behind it.
  • Cliffhanger Wall: Fusion spent 19 years as the final game in the series' timeline, ending with Samus causing the destruction of the B.S.L station and the planet SR-388 in order to stop both the Federation's secret cloning research on Metroids, and the spread of the deadly X Parasites, leaving her future relationship with the Federation unknown beyond an in-universe acknowledgement that she will most likely have to face trial for her actions. During the intervening years between it and the release of Dread, the franchise saw six games in the Metroid Prime sub-series (including a pinball game) and two remakes.
  • Climax Boss: Nightmare. It has been foreshadowed throughout the game, even making appearances in the background of Sector 5 at the end of the first visit, and then it causes the most destruction to the station firsthand, including destroying most of the cooling units of Sector 5 and breaching a water containment unit from Sector 4. It is also arguably the most difficult boss in the entire series much less the game, and its defeat grants you enough defensive ability to reliably take on the rest of the game. As far as its impact on the subsequent narrative goes, you must take a detour through Sector 4 that leads to you unlocking the Level 4 security doors and downloading the Diffusion Missiles, both of which get you chastised by Adam for not following procedure and providing Samus the first major hint that she is being lied to.
  • Clipped-Wing Angel
    • The Core-X left after defeating a boss allows some breathing space. All it can do is ram into you, or attempt to shoot you if it has a beam upgrade. It also spawns X that will heal you if you shoot with the beam, and it only takes a few missiles to destroy completely.
    • When the SA-X takes critical damage, it turns into a towering bug eyed monster... with almost no health left and only a jumping attack that can easily be avoided.
  • Colony Drop: The B.S.L station, at the end, ends up with its orbit altered to intercept SR388. It crashes down on the planet with its self-destruct activated, destroying both it AND the planet.
  • Color-Coded for Your Convenience: The doors are color coded by security clearance. Samus needs to find the override controls for each security level to unlock them. This becomes a plot point, as unlocking the controls also allows the X Parasites to spread further through the station.
  • Console Cameo: The Nintendo GameCube makes a small appearance as part of the junk blocking the air filters in Sector 1.
  • Continuity Nod: Ridley's desiccated corpse appears as a recolor of his Super Metroid sprite before being taken over by an X Parasite and transforming into Neo-Ridley, which uses a completely different sprite.
  • Continuity Snarl: Compared to Metroid II: Return of Samus, the Omega Metroid is portrayed in a completely different manner than in that game. The Metroid II version could fly, breathe fire, and was invulnerable to the Ice Beam and had to be killed by missiles. In Fusion, it can only claw swipe, doesn't fly, and is killed by the Ice Beam. Its later portrayal in Metroid: Samus Returns is more in line with its Fusion counterpart (although it retains its fire-breathing from Metroid II), suggesting that this may have been a deliberate retcon.
  • Cool Starship: Samus' gunship. The loaner ship she gets from the Federation after totaling her own in the opening may count as well.
  • Darker and Edgier: Than the previous games. Super Metroid was darker than the previous series, but this game is almost a Survival Horror game in tone. Samus has to run for her life should she encounter the SA-X, who possesses her upgrades and Ice Beam. Not to mention the Nightmare is one of the most horrifying bosses in a Game Boy Advance game. Even the formerly benevolently portrayed Federation is shown to have shady goals that force Samus to turn on them, which she realizes will make her viewed as an enemy.
  • David Versus Goliath: Many bosses, especially Nightmare, are much bigger and more dangerous than Samus.
  • Developer's Foresight
    • After getting the Varia Suit back, the formerly harmful blue X become harmless like all the other types, but the X (who intentionally suicide bombed you to weaken you) don't know that yet, so rather than immediately starting to run away, the first few blue X you find will continue to suicide bomb you, only running once they see it's stopped working. You can even keep on purposely avoiding the Blue X to prolong the time it takes for them to stop the kamikaze attacks.
    • Various locations on the B.S.L station, such as the Central Reactor Core and the Boiler Room, will be in full operation if visited before their respective crises occur, via sequence breaking. An exception to this is the area outside Nightmare's boss room, which is already ransacked even before his rampage.
    • Through a sequence of Shinesparks, it's possible to make it back to the Navigation Room at the beginning of Sector 4 after defeating Nightmare without going the long way and getting the Diffusion Missiles. Adam and the unknown Federation member do have something to say about this.
  • Differently Dressed Duplicates: The SA-X is a perfect copy of Samus's Varia Suit... Except the SA-X's eyes are blank and white. However, in this game, Samus herself isn't wearing her iconic Varia Suit because the SA-X overtook it, so she's wearing the more organic and streamlined Fusion Suit.
  • Do Wrong, Right: When Samus learns that the Federation is sending a research team to try to capture the X, Samus realizes that most likely the X will instead assimilate the research team and then spread throughout the entire galaxy. When she demands to be let out of the navigation room so she can blow up the station, expressing willingness to go down with the station, ADAM mocks the plan, not just because it would still leave all the X on SR-388, but it would also kill the only person capable of even fighting the X, Samus herself. He then instead proposes to instead alter the station's orbit to include the planet with the explosion, which would also give her time to get to her gunship to escape.
  • Dub Personality Change: Applied to an entire government entity rather than a character in this case. The Galactic Federation in the original Japanese is presented as a complex network of independent branches, while it's addressed as a singular entity in the English localization. This has the side effect of making the entire governing body appear villainous in the English release, as the hidden project to clone and weaponize Metroids and X are attributed to the entire Federation rather than a single potentially rogue branch like it was in the original Japanese. This discrepancy was later fixed in Metroid: Other M but it's only brought up in one line in the game in a very contentious cutscene that some players could overlook.
  • Early Game Hell: The game does a good job of showing Samus' weakened state in the game's opening minutes: even the most basic enemies hit hard, and even though you find a couple of energy tanks fairly quickly, they do little to mitigate the danger, and the SA-X barely has to do more than look at you funny to kill you. It's really only when you find the Varia Suit that you're finally on an even keel with the opposition.
  • Earth-Shattering Kaboom: SR388, when you collide the B.S.L station with the planet. The station's self-destruct had been activated and its orbit had been altered to intercept the planet. The explosion is powerful enough to completely destroy the planet.
  • Evil Is Not a Toy: The Federation seems to think it is, because they first have been trying to weaponize Metroids, which never ends well, and then later on have thoughts on weaponizing the SA-X, which is even less desirable. Samus detonating the station and taking everything therein with it is her attempt at sending this message, but the ending suggests all it will accomplish is making her a wanted fugitive in the eyes of the Federation, who probably doesn't care that they're messing with things way over their head.
  • Evil Versus Evil: The X Parasites and Metroids really don't like each other. Put an SA-X and a Metroid in the same room, and they'll both ignore Samus and go right for each others' throats (or the nearest anatomical equivalent). There's an actual canon reason for this: the Chozo designed the Metroids in the first place to prey on the X, right down to the cellular level (hence why the Metroid vaccine works). After absorbing Samus' abilities, the X can now fight back.
  • Exact Time to Failure: Three times: stopping the boiler room from exploding, escaping the restricted lab after the SA-X begins destroying it and dropping the entire space station onto SR388.
  • Exposition Fairy: Adam serves as this, giving Samus background information on the station along with her objectives and directions throughout the game.
  • Facial Horror: Nightmare's already creepy face will melt as it takes damage.
  • Final-Exam Boss: In a way, the SA-X is one, for you have to use everything you have learned about your upgrades to defeat it, especially if you missed a lot of energy tanks.
  • Fish Person: Many of the X-infected Space Pirates transform into fishlike versions of themselves when in water.
  • Five-Second Foreshadowing: Compared to some of the examples below, at least. The Metroid skins only show up after defeating Neo Ridley, and assuming you follow the story events without deviating to find missing upgrades, you'll encounter the Metroid who shed them not very long after finding the first skin.
  • Foreshadowing: There's lots, all over the game.
    • The elevator from the main deck to the sector elevators has a seemingly meaningless mid-section. At first, it seems like an excuse to show the shaking from the SA-X's Super Missile in the previous cutscene, but it shows up when Samus takes the same elevator back up to find Etecoons and Dachoras. It finally pays off when the power dies and Samus has to break out of the shaft at that midpoint.
    • Even from the very beginning, the fact that the Federation has preserved Metroid DNA comes back in a major way when you discover the Restricted Lab.
    • In Sector 1, Samus will encounter X mimicking Zebesian Space Pirates. Why would X be taking the form of a creature not native to SR388? Good question, and it's the first sign the Federation's work here isn't totally on the up and up.
    • A Chozo statue, previously only found on Chozo-inhabited worlds like Zebes, Tallon IV, and SR388, appears in Sector 1. The same Etecoons and Dachoras from Super appear in the Habitation Deck. Ridley appears frozen in cryogenic storage. The Federation have clearly been scouring Samus' previous exploits, searching for something. Stumbling onto the Restricted Laboratory reveals just what they were looking for, and found.
    • Remember how in Metroid II, Samus was sent to SR388 to dispatch all Metroids? Including some in deep, nocturnal areas of the planet? Is it any coincidence that Sector 1 (SRX), a section designed to replicate the environment of SR388 completely, is located right next to Sector 6 (NOC), a place meant for housing nocturnal creatures? What could be between them? A restricted laboratory, involving the breeding of Metroids, perhaps?
    • The X are presented as mindless creatures, who invade an area, consume all of its organic material, and then proceed to hang around in its place. At first, the X attempt to get themselves absorbed to damage Samus, until they find it only heals her, and the X will start actively fleeing her. It happens again in Sector 6 (NOC), where the X invade Sector 5 (ARC) and deliberately modify their own genetic makeup by hanging around in the cold so that they can damage Samus again (until she re-acquires her Varia Suit). All of this leads up to the reveal that the X are, in fact, intelligent creatures. They still want to infect the entire galaxy, but they can think and come up with plans.
    • Related to the above, if you are unfortunate enough to get caught by the SA-X, you find that its preferred method for dispatching Samus is to freeze her with the Ice Beam, before shooting her with a Super Missile, which demonstrates that the SA-X, and the X in general, are far from mindless if they can find the logical weakness of a being that is part-Metroid, rather than just swarm her.
    • Sector 2 (TRO) has dimmer lighting than the other sectors when you first visit, and in the background, you can see roots growing on and clinging to the sector's systems. Later on, the power goes out completely and you have to find out what's causing it, only to find the reactor core choked by plant roots. Naturally, this means your job is to find the plant responsible and kill it.
    • Samus also discovers Zeroes (larval Kihunters) in Sector 2 (TRO) upon her first visit. After defeating Zazabi they are seen in a pupa state, beginning their final development. Later in the game when Samus returns to the sector, the pupae have hatched and dangerous adult Kihunters are present.
    • At the end of the briefing on Serris in Sector 4, the AI notes that they can't cut the power without causing problems on the station. Nettori's roots end up doing just that after you collect the Power Bombs.
    • After regaining the Varia Suit, the AI mentions that Samus could possibly regain the Plasma Beam from a Core-X, yet expresses surprise when she manages to do exactly that. After Samus leaves the Restricted Laboratory, the AI reveals that the Federation had already finished the Plasma Beam data, and deliberately withheld it from Samus to keep her from engaging the SA-X.
    • Sector 5, on arrival, is rather still and calm. However, near the end of her first visit, shortly before the emergency in Sector 3, Samus will witness a horrific shape flying around in the background. While it's shadowed out and impossible to see, it's clearly extremely agitated. It's Nightmare who, eventually, breaks free and completely wrecks the sector, and Samus has to fight it.
    • Following what was inside the Restricted Lab, Adam will inform you that SRX was an ideal breeding ground to raise Metroids from the larval stage to even the Omega stage of their development, but in the lab itself, all you could see were larval, Alpha, Gamma, and Zeta stages. Once you defeat Ridley, you make your way back through Sector 1 in order to find a navigation room. Along the way, you pass some strange skins... They almost look like Metroid skins for the Larval, Alpha, and Gamma stages... And then during the countdown, you find a Zeta skin inside the hanger you're supposed to find your ship in, just in time for the newly-morphed Omega Metroid to show up.
  • Fridge Logic: In-Universe, at the end of the game, Samus wonders how her ship flew into the hangar after she defeated the Omega Metroid, since it doesn't have an autopilot. She gets an answer shortly after: the Etecoons and Dachoras helped pilot the ship.
  • From a Single Cell:
    • X Parasites, if not absorbed, will simply reform into another body.
    • Also, the Federation had preserved a Metroid cell culture from the Metroid Hatchling. A vaccine made from these Metroid cells was used to cure Samus when she was infected by an X in the game's intro. And they also used it to grow new Metroids in a restricted laboratory hidden on the B.S.L station. Adam claims that this was only for "peaceful applications." This follows from the events of Other M, where the Federation was also breeding Metroids.
  • Game-Breaking Bug: The "TRO Trap" glitch, a result of both a design oversight and a player attempting to sequence break past Yakuza to fight Nettori by using a hard to execute Shinespark maneuver to access the pathway to Nettori without the Space Jump. Thing is, the game is specifically programmed to make you fight bosses and gain items in the exact order the story demands. If you skip Yakuza, he won't return if you come back, meaning you lose the Space Jump. Making things worse is that Nettori won't give you its weapon either—you just get a message saying the level 1 doors are unlocked (a default error message). Both of these completely break the game and make it impossible to complete.
  • Gameplay and Story Segregation: For an extremely intelligent parasite mimicking Samus at full power, the SA-X's Artificial Stupidity make you wonder just how much of a threat it really is. There are times where it can't see you sitting right above or below it, it doesn't chase you through Morph Ball slots despite having the ability, and so on.
  • Generic Doomsday Villain: The X donít seem to have any goals or motivations beyond reproducing. In a way, this actually makes them even scarier as they canít be reasoned with and it sells the Eldritch nature of the species.
  • Genre Mashup: Part Sci-Fi Action-Adventure, as is typical of the franchise, part Zombie Apocalypse Survival Horror.
  • "Get Back Here!" Boss: Serris, to some extent, is this. You're stuck on four platforms over water, while you have a giant armored snake charging at lightning speed in and out of the water, only exposing its vulnerable head at brief moments. Don't bother trying to chase it, though, as you move at a slug's pace in the water, and Serris can easily run you down to take a few bites out of your life bar. The only effective way to fight it is to stay on the platforms and wait for it to appear. Or stay crouched in the water, just to the left of the rightmost spot where Serris comes into / out of the floor. It never hits you there, and you have a shot at its head when it comes out/goes in.
  • Giant Space Flea from Nowhere: Averted for the most part as most of the bosses are given decent foreshadowing but this is played straight with one boss, Zazabi. You just run into him and have a boss fight and it's never explained what it is besides something the X Parasites took over.
  • Glass Cannon: Without her proper classic Power Suit, Samus is more vulnerable here than in any prior game, and even slow-moving, standard enemies can take upwards to nearly half an energy tank in a single touch of Collision Damage. Of course, her firepower tears through most normal enemies in kind, and even a few of the weaker bosses, giving her a surprising amount of kick even for a lesser state.
  • The Goomba: Hornoads and Zombies on the main deck and Sector 1 are the most common enemies you face early on. The Zombies in particular are the most pathetic enemies in the game, dying in one hit from your default blaster and having a very short attack range that barely deals any damage.
  • Graying Morality: This game paints the Galactic Federation as becoming as Unfettered as the Space Pirates (though in the Japanese version, it's said to only be a subset of the group). Over the course of the game it's revealed they have been weaponizing Metroids (for "peaceful applications", though Samus doesn't buy it since she knows how uncontrollable they can be) and that, based on the events on the B.S.L station, the X Parasites (and SA-X specimen specifically) could also be potentially weaponized (which, again, sends Samus sideways that they would think that trying to cultivate such things wouldn't be a disaster waiting to happen. Not helped by the revelation that they were deliberately withholding upgrades from her so she couldn't kill the SA-X). She destroys their research in order to prevent any of this from coming to pass, but expects that she will be branded a traitor — though Adam insists that at least one of them will understand.
  • Guide Dang It!
    • After you beat Nightmare, you have to go to an underwater section of Sector 4 that you were cut off from before, and there's a spot where it's easy to get stuck due to the game only giving you an obtuse hint via some traveling enemies that you're supposed to Power Bomb one of the glass tubes to access a crucial area. Shortly after in the same area is a small maze that tends to trip people up because you have to go through a non-destructible fake wall to progress, something that's been exclusively reserved for secret areas and items up until this point, and the only clue to this, a fish swimming through said wall, will almost certainly not be seen if you used a Power Bomb to search for a breakable wall.
    • The Charge Beam, when fired, has a large flare effect around the Arm Cannon which also does damage, meaning shooting an enemy with it at point-blank range deals more damage than just hitting them with the Charge Beam from a distance. This is never brought up in the manual and was not a mechanic in any Metroid game before or after this one, and the only indications of this technique in gameplay are the aforementioned flare, which is easy to mistake for just a visual effect, and a sparkle-like sound that plays when you hit a point-blank charge shot. While this is never required for anything, knowing this can let you take down bosses much quicker than normal - Serris, for example, can be taken to his Core-X form with just two point-blank shots.
  • Guilt-Free Extermination War: The game ends with Samus eradicating all the X Parasites on the station and on SR388, but given how they would likely lay waste to the entire galaxy if allowed to escape, itís justified.
  • Hailfire Peaks: Every zone is "X meets Eternal Engine," since you have various elemental environments taking place aboard a space station. Occasionally, you'll see several machines in the background. For Sector 3 in particular, the "X" is Shifting Sand Land for the upper rooms and Lethal Lava Land for the lower rooms.
  • Heinousness Retcon: Ridley started as one of the many Space Pirates bosses of the original games, and this remained unchanged for a time. There wasn't any connection between him and Samus. However, due partially to his popularity as his role expanded two adaptations rewrote him to give him a much more personal connection. The Japanese exclusive child mode of Fusion was the first to show Ridley leading the Space Pirate raid that destroyed Samus' home planet and leaving her an orphan.
  • Hell Is That Noise
    • The footsteps of the SA-X can become Paranoia Fuel really quickly.
    • The buzzing of the Ki-Hunters will become paranoia fuel as well, especially during the sequences immediately before and after battling Yakuza / Gedo, when save points are few and far between, you're low on health, and they take several hits to kill with your Wide Beam.
    • The strange howling noise that Nightmare makes, not helped by the fact that it also causes Paranoia Fuel during your first trip to Sector 5 when you see Nightmare's shadow flying through its containment tank every so often. Then on your second trip to Sector 5 the ''absence'' of the noise becomes the scariest thing about it as you see that Nightmare has broken out of its containment tank and is nowhere to be found.
    • The most infamous noise though, is courtesy of Neo-Ridley. Upon transforming into his X form, he greets you with a deafening shriek that is loud to the point where the GBA lags just playing the audio clip.
  • Heroic Sacrifice: Samus almost attempts one by detonating the self-destruct charges on the station to destroy the X Parasites aboard; herself included. The AI Adam, however, informs her that doing that would be stupid, as it would not destroy the X on SR388, and only serve to destroy the X's greatest obstacle: Samus. A different plan is then created, which destroys all the X in both locations and allows Samus time enough to escape the station.
  • Hero with Bad Publicity: The game ends with Samus speculating she will become this, having destroyed a Galactic Federation space station and the planet it was orbiting, and the galaxy unaware as to why this seemingly terrorist act was necessary to stop the very dangerous threat of the X Parasites.
  • High-Voltage Death: Electrified water is a hazard in Sector 4, AQA, that damages you over time. The player can accidentally kill themselves falling into it if they have low health. You encounter this again during the second battle against the B.O.X.; as long as its in contact with the water flooding the area, it is electrified and you will take damage as long as you're in it.
  • Hitbox Dissonance
    • There is one point in the game where the hitbox does not exist. If you willingly Troll the SA-X, there is one point where if you stand on a particular root, Samus will stand in just the right position that the Ice Beam will be fired behind her hitbox, completely missing.
    • A mild example with Nightmare: even when Nightmare's face is grotesquely deformed to half its size, the entire opening in Nightmare's armor can be shot to deal damage.
  • Holding Back the Phlebotinum: Near the end of the game, it's revealed that the Federation was deliberately withholding some of Samus' more powerful upgrades to keep her from killing the SA-X. The Plasma Beam was ready but not sent to a Data Room, while Diffusion Missile data had already been sent, and Samus managed to get it despite not being told of it.
  • Hopeless Boss Fight
    • Every SA-X encounter before the proper boss battle in the finale. You have to either evade or escape it.
    • The final boss battle against the Omega Metroid starts out like this as well, similar to the final battle with Mother Brain in Super Metroid, until an SA-X attempts to fight it and is reduced into its Core-X form, which Samus absorbs to power up and get the Ice Beam.
  • Hub Level: The Main Deck of the station, as it contains elevators to each of the 6 sectors of B.S.L.
  • Ice Crystals: Sector 5 contains these in some of the backgrounds, but when Nightmare breaks loose and wrecks shop, they disappear.
  • I'm Melting!: When Nightmare's mask is removed, you can use missiles and charge shots to mash its face into mush.
  • Implacable Man: The SA-X, especially in the beginning. Suited up with the Varia mode Chozo Power Armor, armed with Super Missiles, Power Bombs, the Screw Attack and the Ice Beam. The best you can do at first is just run. You can fare a little bit better once you get the Ice Missiles, which slow it down, but you still can't actually hurt it until the final boss fight.
  • Insectoid Aliens: The "Kihunters" are insectile in appearance, having wings and a carapace. They also start off as caterpillar-like creatures that turn into pupae before turning into them.
  • Invincible Boogeymen: The SA-X of is a copy of Samus at her full strength, and as such can kill her in a few hits, freeze her in place and generally forces the player to run when its telltale footsteps show up.
  • It Can Think: The X are definitely intelligent; one of the X Parasites mimics a scientist and attempts to melt down the boiler room in an attempt to destroy Samus and all the X on board, but preserve the rest of the species on SR388. The X also have shown that they possess enough intelligence to deliberately modify their genetic makeup in an attempt to try to freeze Samus. However, they are still ultimately driven by a mindless desire to consume everything.
  • It's The Only Way To Be Sure: Adam persuades Samus that simply activating the self-destruct system on the B.S.L station isn't enough, and it's what the X want, because it spares their brethren on the planet below. Instead, the AI implements a plan to change the orbit of the moon into a collision course with SR388. The blast would eradicate the X (indeed the impact and detonation destroys the planet).
  • Kaizo Trap: In the game's finale, it is possible to defeat the Omega Metroid, then have the timer run out while Samus's Gunship is docking.
  • Kill and Replace: The X mainly infect lifeforms to absorb their DNA and memories while killing their host, which they can then perfectly replicate. This notably happened to the crew of the B.S.L station and many other creatures, to the point that all (save B.O.X. and the Omega Metroid) of the enemies Samus faces in-game are mimics created by the X as opposed to being mutated outright.
  • Late-Arrival Spoiler: The ship's AI indeed being the uploaded mind of Samus' old commanding officer, Adam Malkovich. The reveal shocks Samus and was meant to be a lategame twist; these days, it's very hard to discuss this game or later Metroid games without talking about this plot point.
  • LEGO Genetics
    • Samus is a human being genetically enhanced by Chozo blood and, by this game's intro, she's also part-Metroid.
    • Technically the X count as well, due to their trait of retaining useful genetic information from past hosts and "improving" their current ones. It happens often in gameplay, where X from one defeated host will infest another and change its abilities. Horrifyingly, it also counts for the SA-X, as when it mutates after being defeated the first time, it takes on traits from multiple species to try and kill Samus.
  • Leitmotif
    • The SA-X has two of its own, for when you haven't alerted it yet and when you have and are running. It gets incorporated into its boss theme.
    • Most bosses have their own themes, with the exception of the Beam Core-Xs encountered through the game, Nettori/Barrier Core-X, and Serris/Yakuza.
  • Let X Be the Unknown: The gelatinous X Parasites.
  • Literally Shattered Lives: Implied Trope. The SA-X, believing Samus to be a Metroid, will try to freeze her with the Ice Beam and kill her with a Super Missile. However, because Player Death Is Dramatic, we never get to see this.
  • Lone Wolf Boss: The B.O.X. Security Robot isn't affiliated with the X Parasites—it's just a security feature of the station gone awry. Samus unwittingly allows it to become infected by the X by damaging it enough to expose its organic neural core, which prompts a rematch with it later on.
  • Lost in Translation: In the original Japanese script, the Galactic Federation is a complex network consisting of multiple branches. This was not carried over into the localization, making the Federation feel like a singular monolith, which would be the Federation's characterization in the English version of titles going forward. This has the effect of making the Federation as a whole appear much more villainous in the English version as the secret project to weaponize the Metroids and X is attributed to the entire Federation rather than a singular branch that may have not been operating with the knowledge of the rest of the Federation.
  • Lovecraft Lite: Fusion is a lot more overt with its horror elements than past Metroid games. The main antagonists are parasites that enter a host's body by phasing through their skin, killing them from the inside and effectively taking over their bodies. The B.S.L station has a much more suffocating atmosphere compared to previous Metroid games, thanks partially through the use of Railroading and the fact that Samus' weakened armor makes her take far more damage than previous Metroid games. Then there's the sheer horrifying creatures that prevail throughout the game, be it Nightmare or the Implacable Man that is the SA-X, an X mimicking Samus in her prime that Samus can't even hope to beat at the start of the game, resulting in many tense chase sequences that Samus escapes only by the skin of her teeth. However, despite this all, it never descends into outright Survival Horror, as Samus is still very much capable of holding her own against the X and ultimately gains enough upgrades to finally take on and kill the SA-X.
  • Man-Eating Plant: The Samus-Eaters that are present in Nettori's room. Land in one of them, and you will take damage until you can get out.
  • Meaningful Name: Nettori, which shares the last four letters of its name with the first four of Torizo, the Chozo statues that attacked you in Super Metroid. If you look closely at Nettori's main biomass, you may notice that it looks exactly like a Chozo Statue being tangled up in plants.
  • Meat-Sack Robot: Nightmare and the B.O.X. security robot. The former is a gravity-manipulating robot with organic components (including a six-eyed melting face with green skin). The latter is an armored security bot that contains an organic brain as part of its AI's neural network.
  • Merchandising the Monster: Beating the Japanese version of Fusion on Hard Mode with only 1% of the expansions collected nets you an ending image showing toy 2-packs that contain action figures of both Samus Aran and the SA-X that hunted her for the entire game.invoked
  • Mimic Species: X Parasites work by assimilating and then replicating whatever they infect to a near indistinguishable degree once they get used to how a host works.
  • Monochromatic Eyes: The SA-X has blank white eyes, as seen after it blows up the elevator and more clearly in the game's commercial.
  • Monster Delay: Nightmare is introduced as it streaks across Sector 5 of the ship. Mission Control tells Samus to ignore it for now, so she continues on. Later, the AI realizes how much damage it has caused and tells Samus to destroy it.
  • Names to Run Away from Really Fast: Nightmare. Its name alone can inspire terror, and its appearance and power allow it to live up to its name.
  • New World Tease:
    • You can get an early glimpse of the lower section of Sector 4, but you won't be getting anywhere due to some unkillable enemies blocking your path.
    • After your first visit to Sector 3, you can visit Sector 5 early, but you won't be getting far in it due to the Level 3 Security Hatches and the fact that Samus can't survive the extremely low temperatures of most of it without the Varia Suit upgrade from Sector 6, and can't use her Speed Booster due to the cold literally knocking her off her feet from the damage it inflicts.
  • Nice Job Breaking It, Hero
    • Samus' Metroid extermination campaign caused an explosion in the population of the X Parasites, which the Metroids had been designed to kill. Oops.
    • Overriding the security doors allows the X to spread through sectors that they had previously been kept out of, but Adam always makes you aware of that fact before he sends you to unlock that level of door.
  • Nice Job Fixing It, Villain: Had the X left Nightmare and especially B.O.X. alone, Samus wouldn't have wandered off the path the AI planned for her and would have been likely still in the dark about the Federation's attempts to weaponize the X and cloned Metroids until it was too late. Samus defeating B.O.X. and getting the Wave Beam grants her access to the Restricted Laboratory, which leads directly to her and Adam crashing the B.S.L station into SR388 to kill all the X.
  • Nightmare Face: Fittingly, Nightmare's face. Once its faceplate is broken off, it reveals a green, gloppy, melting mess with six eyes and an anguished expression on its mouth. It doesn't get any better as the fight goes on, with the green turning to red and the face continuing to melt out of its socket as it takes more damage.
  • Nintendo Hard: When the A.I. warns Samus she was significantly weakened from the X Parasites and the Fusion Suit is far less protective than her normal suit, it's not kidding. Even fully upgraded, Samus takes the highest amounts of damage in this game than any other Metroid game by far. This means you have to be far more careful and strategic even with basic enemies, and going in Rambo-style like in other Metroid games is highly likely to get you killed. And that's not even going into the game's bosses, which there are more of than any 2D Metroid game to date.
  • Nostalgia Level
    • Sector 1 was designed to be a perfect replica of the environment of the planet SR388, where Metroid II takes place.
    • Some hidden parts of Sector 1 have a stunning resemblance to Ridley's Lair and Tourian, and they should, because you fight Ridley-X there.
    • The Secret Laboratory's escape sequence is very much like the ones in Metroid and Super Metroid.
  • Nothing Is Scarier: On your first visit to Sector 5, you'll enter a massive room with a glass window background, and every few seconds, the silhouette of something BIG will quickly fly past, shaking the room as it does so. The next time you come back, the glass is shattered... and that big thing is nowhere to be found.
    • During the section when the power is out, the player likely expects the SA-X to show up any second. It doesn't, at least not until shortly after you defeat the boss responsible for the power outage.
  • Oh, Crap!
    • The Blue X actually manage to pull this off despite not having faces. When moving through Sector 6 to get the Varia Suit, they actively target Samus. After getting the suit, the first bunch that see her attempt to attack. The ones that aren't absorbed stop short and immediately try to fly away upon realizing they no longer harm her.
    • When Samus returns to a Navigation Room after restoring the Plasma Beam, the single weapon in her entire arsenal that's been established to be capable of combating the SA-X, the AI takes notable pause before expressing its surprise and telling her to continuing to avoid the SA-X anyway as it may have learned new tricks itself. The reality is that by this point the Federation had long since finished the Plasma Beam themselves and decided to keep it from Samus because they're interested in the SA-X themselves and didn't want her capable of destroying it.
    • Samus lets out one when the computer tells her "one of the SA-X is on its way here".
      Samus: ONE? ...Are you saying there is more than one SA-X?
  • One-Hit-Point Wonder: Playing the game without collecting a single energy tank (as part of a Minimalist Run) quickly turns Samus into this. Most enemies take away huge amounts of health either by attack or contact, and many of them, including the boss fights, can kill her in one hit.
  • One-Letter Name: The X Parasites are typically just called "X" just to shorten it. Even the Chozo called it that.
  • Organic Technology: The security robot has an organic CPU, which the X Parasites infect in the later half of the game. Samus' suit is also apparently organic in nature, as the X Parasites are able to infect and then replicate it. There also exists the Nightmare, a massive creature who can mainipulate gravity and is explicitly stated to be a bio-mechanical organism.
  • Our Zombies Are Different: Everything the X infect dies, and the X are now able to clone what they absorb, creating zombie-like clones of dangerous enemies.
  • Patchwork Map: The game takes place on a biological research station, and in order to support the needs of the wide variety of fauna and flora on board, each of the six sectors has its own artificially created environment.
  • Perspective Flip
    • In Metroid II, Samus was tasked with eradicating all of the Metroids for the good of her species. In Metroid Fusion, the SA-X is attempting to wipe out all Metroids - including Samus - for the good of its species. This time, Samus has become a sort of Human-Metroid Fusion.
    • This works on a meta-level as well. The SA-X is mimicking Samus at her full power, in other words, how she more or less was at the end of Super Metroid. In that game, you, the player, were that nigh-invincible juggernaut. Fusion flips the script by giving you a depowered Samus that has to fight the very same nigh-invincible juggernaut that was once your Player Character. Naturally, you're virtually helpless against it for most of the game.
  • Plug 'n' Play Technology: The Fusion Suit is able to form new abilities just by absorbing "data" acquired from Data Rooms and the DNA of the X Parasites.
  • Point of No Return: Once you go up to the main deck to alter the station's trajectory, every possible route back to the various sectors is sealed off. Your only option left is to finish the game. Except in the Endgame+, which does not seal off the sectors until the final evacuation countdown.
  • Post-Defeat Explosion Chain: Bosses are consumed in explosions before pixellizing into their Core-X forms.
  • Posthumous Character: Adam Malkovich, an old Commanding Officer who Samus talks about serving under in the past. She's actually serving under him in this game too.
  • Power Copying: Samus gains abilities by defeating mimicked bosses and absorbing their X-parasites, though she does get some abilities from Data Rooms as well. For the most part, each X incorporates their item's power into its boss moveset.
    • Morph Ball from Arachnus; it retains its ability to curl up into a ball from Return of Samus.
    • Charge Beam from the fake Chozo statue; Charge Beam shots are fired by the Core-X when it reveals itself.
    • High Jump and Jump Ball from Zazabi; it jumps high into the air to land on Samus.
    • Speed Boost from Serris; it enters an invincible hyper-speed state every time it takes damage.
    • Varia Suit from Barrier Core-X, which had actually stolen it from a data room; it's the biggest and most defensive Core-X and the only one immune to missiles at first.
    • Wide Beam from a scientist; it turns into a Core-X that fires Wide Beam shots.
    • Space Jump from Yakuza; once it loses its legs, it hops around in thin air to keep fighting.
    • Plasma Beam from Nettori; both the Chozo Statue-like body and the Core-X fire Plasma Beam shots.
    • Gravity Suit from Nightmare; it supplements the Gravity Master tactics it uses in the first part of the battle.
    • Wave Beam from the Security Robot B.O.X. (Fight 2); it becomes a Core-X that uses Wave Beam once its body is dismantled.
    • Screw Attack from Neo Ridley, although this boss doesn't really use the attack unlike the others.
    • Ice Beam from SA-X, which uses it as its main weapon.
  • Power Pincers: The X-mimicked Space Pirates have these, but they only use them to shoot lasers.
  • Puny Earthlings: The X don't have much interest in mimicking the B.S.L human crew, since there were no armed soldiers present at the time of its outbreak, and the scientists lack any offensive capabilities the parasitic organisms can make use of. However, as they realize Samus is a potential threat to their species on the planet, they try to change strategies, mimicking a deceased crew member to destroy the station with bounty hunter aboard. Their knowledge wasn't perfect though, and their meddling with the main boiler tripped the Emergency Broadcast, alerting Samus and Adam to their plot.
  • Pupating Peril: Zeroes in Sector 2, slow-moving caterpillars that start pupating once you beat the boss. When you return during the blackout, they have become the flying Kihunters.
  • Rainbow Speak: The game makes use of colored text throughout Samus's briefings. General hints and things of note like items are in yellow, while location names are in purple. All four Security Rooms also use their respective colors (blue for level 1, green for 2, yellow for 3, and red for 4) when they're brought up in dialogue.
  • Raising the Steaks: The X not only infects people, but also alien wildlife on the B.S.L station.
  • Reasonable Authority Figure: Adam turns out to be one, and in the Bittersweet Ending he offers hope that there may be others out there who understand why Samus destroyed the station.
  • Recurring Boss: The Security Robot B.O.X.. You fight it twice (more than any other boss in the game), and it changes its weaponry and tactics for the second fight.
  • Regional Bonus: The Japanese version, which was released after other regions, adds several things to the game. Most of them would appear in all regions come this game's followup, Metroid: Zero Mission:
    • There's an Easy difficulty and an unlockable Hard difficulty.
    • Beating the game unlocks a gallery mode to view the ending images you've obtained on the cartridge.
    • There are eight additional ending images; some of them are earned by completing the game on "Child Mode" (which tells the story without kanji to make it easier for younger players to read), some are earned for playing on Hard difficulty, and some are earned for doing Minimalist Runs. Notably, the "Child Mode" endings reference events that would later be expanded on in the Metroid (Manga). note invoked
  • Revenge: After being chased by the SA-X the entire game, it's only fair that you get your revenge before destroying the B.S.L station.
  • Run or Die: This is the only advice Adam gives Samus regarding SA-X at the beginning, because it's equipped with all of Samus's weapons and abilities, including the Ice Beam, which Samus is now weak against due to the Metroid Vaccine.
  • Scenery Gorn: Sector 5 (ARC), after Nightmare runs loose, looks like a complete and total mess. Broken windows, smashed passages, new crudely made tunnels where there were none before, and rubble scattered all over the place.
  • Self-Duplication: The SA-X. Since the X Parasites multiply asexually, by the time you finally see an SA-X die, there are still ten other Samus copies running around the space station! Fortunately, you only meet one at a time.
  • Senseless Sacrifice: Samus's plan to blow up the space station would have been this, if Adam hadn't stopped her — it wouldn't destroy the X on SR388, and since Samus herself would be now dead, there'd be nothing left to stop them from invading the universe.
  • Sensory Abuse: This game's iteration of Ridley is known for his piercing scream. If you just so happen to be wearing ear buds when he lets it out, prepare to go deaf.
    • Even worse, every time Ridley screams, the game LAGS!
  • Sequence Breaking: The game is mostly designed to make sure you can't do this due to its more linear, story driven nature, but there are a few exceptions, mostly reserved for minor situations:
    • There is a secret conversation that can be seen by using a series of tricky Shinesparks in Sector 4. This almost allows you to skip the Diffusion Missiles; the developers thought to lock the door to the Main Deck. There's also a way to do it without the Shinespark—Samus can freeze the enemy blocking her path by using a very carefully timed and aimed Ice Missile launched from a diagonal angle. The secret message only acknowledges the Shinespark method though, as the second trick was a dev team oversight.
    • It is possible to exploit a glitch in the game's memory that allows you to spawn right in the area before you fight the Omega Metroid. Unfortunately, this breaks the game's event value scripting, so you won't get the Ice Beam back from the SA-X, and the Omega Metroid just stands there, impossible for you to kill.
    • For 0% runs, the hardest sequence break to pull off in the entire game is using the Shinespark in Sector 3 to bypass an organic barrier that can otherwise only be destroyed with Super Missiles and thus avoid the mandatory missile tank on the normal path. How hard is it? It's only possible by pausing to get precision control of Samus' movement for every individual frame, and while it can technically be done without tool assisting, the Shinespark only lasts three seconds and the fastest it's mechanically possible to get into position for the necessary Shinespark is 2.88 seconds (173 frames), so you need frame perfect timing and have just seven frames of error to pull the entire sequence off.
    • Using quick maneuvers, Wall Jumps and Shinesparks, numerous Energy, Missile and Power Bomb Tanks can be obtained without the otherwise required equipment. For example, the Shinespark can be used to bypass a very powerful organic barrier (that could otherwise only be destroyed with a Power Bomb) near the beginning of the game to access a Power Bomb tank—unfortunately, you can't use it until the game gives you the actual ability.
    • When you first enter Sector 3, if you have enough energy tanks on hand, you can go through a superheated tunnel early on and survive before getting the Varia Suit, and if you find out how to get through the next room, your reward is an early missile upgrade and energy tank. This also doubles as a Self-Imposed Challenge.
    • Likewise, if you have enough Energy Tanks and go back to the lava room in Sector 1, Samus can Morph Ball below and grab the missile upgrade without the Gravity Suit.
  • Sequel Hook: Fusion ends with Samus having illegally blown up a Federation outpost and become something way other than human. Sure, Samus had her reasons, but all the implications in-game are that the Federation will be antagonistic with her now.
  • Shotguns Are Just Better: The Wide Beam and Plasma Beam have much larger spread and more damage. The game would be near impossible without the Plasma Beam and actually impossible without the Wave Beam.
  • Shout-Out:
    • A couple to the Stanley Kubrick film 2001: A Space Odyssey, as Adam bears a striking resemblance to HAL 9000. Also, the scene where Adam locks Samus inside the Sector 1 Navigation Room, is very similar to the famous scene from 2001 where HAL locks David Bowman out of the USS Discovery, even down to the dialogue.
    • An unknown shapeshifting alien species steals its hosts' bodies and turns them into Body Horror flesh monsters when exposed? Sounds like The Thing.
  • Sliding Scale of Gameplay and Story Integration
    • Integration: when it's mentioned that the Fusion Suit is weaker than Samus' usual Powered Armor, the game means it. Samus takes ludicrous damage in this game compared to any other Metroid game, even when fully upgraded. In the final confrontation with the SA-X, the SA-X's ice beam will still do about two health bars worth of damage if you're hit by it, despite being upgraded to the Gravity Suit by that point. The plus side is that, since the Fusion Suit is so much lighter, Samus can grab onto and climb up ledges on her own, something she requires a powerup to do in her heavier Power Suit in the later Zero Mission.
    • Segregation: after you finish your first visit to Sector 3, Adam tells you that all of the available Data Rooms she's already visited have been destroyed, sending Samus on a goose chase to use whatever other Data Rooms are possibly available. Thing is, the Sector 2 Data Room remains intact for the entirety of the game, although you're cut off from accessing it again due to the SA-X blowing up the door to it—that is, until you beat Nettori, allowing you to backtrack all the way to the Data Room by the same route you used to escape from it, where you'll find that it's perfectly intact, but never used for the rest of the game. Adam did mention earlier that SA-X activity in the sector made it too big of a risk to go back to it, but it's not clear if he's referring to the destroyed entrance to it or the SA-X being there again (although it does show up in the sector again before you fight Nettori).
  • Something Only They Would Say: How Samus discovers that her AI CO, which she nicknamed Adam, after her old CO Adam Malkovich, actually is Adam Malkovich; after she calls him "Adam" and they talk about Adam Malkovich, he ends the briefing with "Any objections, Lady?".
  • Sound of No Damage: When you use a weapon that's too weak to damage an enemy (or something else, like using a beam on the missile hatch in the opening), you simply receive a small "beep".
  • Space Sector: The sectors on the station are designated numerically then by their environment, for example Sector 2 TRO simulates a tropical environment.
  • Spiritual Antithesis: The dev team intentionally designed Fusion to be the opposite of its predecessor, Super Metroid. While that game was very open, designed around exploration, and had minimal dialogue and storytelling, this game is linear and doesn't open up for a good chunk of the story, while there are many scenes where Samus monologues or listens to the AI guiding her.
  • Suddenly Speaking: Though Samus has extensive internal monologues, she never actually speaks to another character at any point in the franchise, until a scene close to the end of this game, where she verbally communicates on-screen for the first time; a remarkably mundane example where she simply asks for clarification that there are now multiple SA-X parasites.
  • Suicidal Overconfidence: The first few free floating X Parasites encountered will attempt to infect Samus again, not knowing she's now part Metroid and can eat them on contact. They learn to avoid her quickly, but it takes another lesson for the cold X Parasites after Samus regains the Varia Suit.
  • Surprise Creepy: Metroid games are not unknown for having a dark, desolate atmosphere, and Fusion is no exception. What is unconventional for the series is the extent to which the game incorporates Survival Horror elements. Samus's SA-X clone will pursue her relentlessly, and until much later in the game, there is no hope of fighting it - the only available choices are to run and hide.
  • Terraform: The B.S.L space station is actually made from an asteroid moon that orbits planet SR388. The Federation recreates all the typical environments found on the world within it, thereby making a safer place for the researchers to do their work in studying the exotic alien creatures.
  • Too Dumb to Live: The Galactic Federation thinks that the rapidly reproducing parasite that kills indiscriminately, takes the form and abilities of their host, and of which a basic variety nearly curbstomped Samus, would make for an amazing superweapon. And that's not even getting into how that includes the SA-X, a perfect copy of Samus of which there are at least ten of.
  • Unintentionally Unwinnable: The TRO Trap, which is a very tricky Sequence Break that requires you to Shinespark in the Reactor Core to access the path that takes you to Sector 2. This lets you fight Nettori (the root boss) before defeating Yakuza (the suplexing spider), but if you defeat Nettori, absorbing its Core-X will give you an error message saying Level 1 hatches are unlocked instead of the Plasma Beam, and Yakuza will be gone if you try to go and fight it afterwards. The unintentional part arises because unlike other Metroid games, which either encourage sequence breaking or are capable of handling unintended sequence breaks, Fusion is programmed specifically to follow the story's events in order, and as such skipping an entire boss throws progression out of whack in a way the game can't recover from without reloading your save.
  • Unskilled, but Strong: The X's main threat factor is their ability to infect beings with no resistance to them, devour them, and then use their stolen genetic makeup to impersonate those creatures to get close to other creatures, lather rinse repeat. They will also modify that genetic makeup to allow the creature they are impersonating to better defend themselves against actual threats. The X can also consider the greater picture when it comes to their species, and will attempt to sacrifice themselves to preserve the whole, and over the course of the game, they attempt to destroy the whole station just to bring Samus down with them. The X are clearly presented with intellectual capacity and the ability to adapt or make plans more to benefit their long-term survival. This all being said, while the X display intellectual capacity, the SA-X... doesn't. It has all of Samus' abilities as she had in Super Metroid (minus the Hyper Beam), but it has next to none of her intelligence, mindlessly chasing down Samus whenever it finds her, with the closest to intellect it displays being that it will freeze Samus with the Ice Beam and then blast her to smithereens with a Super Missile, calling back to the most effective way to destroy a Metroid in Super. This manifests in its boss fight, where all it does is chase Samus around the room, completely oblivious to any strategy of Samus's, still trying to freeze and destroy her. Samus herself muses about what would happen if the SA-X realized its potential, something the X are clearly capable of. Also, albeit unintentionally, its programming is notoriously poor.
  • Victor Gains Loser's Powers: Samus had all her abilities copied by the X. After killing X-infected bosses (which tend to be able to produce similar attacks), she can absorb the X Parasite to regain that power.
  • Video Game Cruelty Potential: Some puzzles for item pickups require you to let X Parasites infect background creatures and turn them into enemies, just so you can kill them to open a door. For example, in Sector 4 (AQA), one puzzle in the underwater section requires you to let the X turn some docile Scisers into hostile golden Scisers.
  • Villainous Rescue: Samus has beaten an SA-X, plotted the B.S.L station on a collision course with SR388, and is on her way to her ship to escape. But when she gets to the docking bays, the place is a mess, her ship is missing, and there is a huge shed Metroid skin on the floor. Suddenly, an Omega Metroid comes in and screeches at Samus. With one claw swipe, Samus is knocked down to one health, immobilized and completely helpless against the monstrosity... until the SA-X she defeated jumps in and tries to take it down. The Metroid quickly kills the SA-X, but this allows Samus to absorb its Core-X, regain the Ice Beam and kill the Metroid herself.
  • Virtual Sidekick: Samus is guided by an AI version of her former Commanding Officer, Adam.
  • Warm-Up Boss: Arachnus (from Metroid II: Return of Samus) makes a return as the first boss, but is a much more straightforward fight than its natural counterpart. It is also the only boss that takes damage from the normal, non-charged beam.
  • Wham Line
    • "You know that the X reproduce by asexual division. Because of this mechanism, there are now no fewer than 10 SA-X aboard the station."
    • "Any objections, Lady?" Samus even gets an Eye Take in response.
  • Wham Shot
    • As Samus takes the main elevator down for the first time... and then something blasts through the wall, and a copy of Samus in her old power armor walks in, followed by a close-up on its Blank White Eyes.
    • Accessing the Restricted Laboratory after picking up the Wave Beam and stumbling across the Metroid breeding tank.
  • What the Hell, Hero?: Late in the game, Adam will chastise Samus for bypassing Security Level 4 without consulting HQ and later, blowing up the restricted lab containing Metroids.
  • Whole-Plot Reference: After investigating a remote outpost, a research team discovers a lifeform which assimilates, infects, and transforms and/or replaces its victims. Armed with this knowledge, one of the last survivors destroys the facility rather than allow the creatures to live and spread beyond it.
  • Wrong-Name Outburst: Having mentally nicknamed her AI handler "Adam" after a former CO, Samus accidentally calls it by that name out loud during an argument over whether to allow the Federation to capture and study the X Parasites. This leads to The Reveal that the AI really is Adam, courtesy of Brain Uploading.
  • Zombie Apocalypse: The B.S.L station succumbs to one courtesy of the X Parasites. It's a small taste of what could have happened to the entire galaxy had they got off the station. They kill whatever they infect and either pilot the bodies while applying Body Horror or simply mimic them.


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