History VideoGame / MetalSlug

* TrickBomb: Aside from the standard stick grenade, there's also the [[MolotovCocktail Fire Bomb]] and Stone. The regular grenade explodes in a vertical burst that does good damage to machines and kill several on-foot enemies in one go. Fire Bomb explodes in a spread of flames, making it good against swarms of on-foot enemies but not on machines. Stone can only hit one target at a time, so it's not good on on-foot enemies, but does surprisingly heavier damage against machines.

* PrecisionFStrike: [[https://youtu.be/2W1D-ZotM9E?t=83 It sounds like Tarma is letting one out upon dying]], which is supported by the fact that the [[Bowdlerise bowdlerised]] [=PS1=] release of ''X'' replaces his death scream with Marco's. This can quickly turn into ClusterFBomb given the game's NintendoHard nature, which makes his aversion to swears in story-driven installments such as ''3D'' and the tower defense games all the more surprising.

The game's rather threadbare story usually involves the general Donald Morden, who is leading a rebel army to take control of the world. Over the course of the series, he makes deals with rebels, mad scientists, demons, and even aliens in his bid to change the world.

* AllThereInTheManual: It is only via supplemental materials do you find out the names of lots of things, like the end-mission bosses in the games (sans Morden, probably), that explorer in ''2/X'' (Scott Amundsen Jr.), that old man whose briefcase you can take for points (Duke Koudou), and that girl at the end credits of ''1'''s single-player mode who's mourning a lost love (Satiko Suzuki).

** Morden, however, suffers from his eyepatch being worn on his left eye rather than his right, which he spends the entirety of the first game doing. ''Metal Slug 3'' turns this into a plot point: the fake Morden reuses the sprites from the first game while the real Morden has new sprites with his eyepatch in the correct position.

** '''UH-OH BIG!'''
*** '''MISHOWN COMEPLEET''' ''[ending theme]''

* ArmiesAreEvil: General Morden's Rebel Army show up in just about every game, and their job is to cause [[ForTheEvulz chaos and destruction as]] [[AllThereInTheManual payback for the death of Morden's wife and daughter]]. There are several other, less important armies as well -- and you know they're evil because they shoot at you and not Morden's rebels -- like the Arabian Infantry (found early on in ''Metal Slug 2'' and ''Metal Slug X''), the Ptolemaic Army (your main opponents in ''Metal Slug 5''), and the Division 6 of the Japanese Army (found only on a secret path in ''Metal Slug 3'').

** ''Metal Slug 7'' and its remastered version ''XX'' has pre-rendered background instead of being hand drawn, while averted for the rest of the characters and vehicles.

** Subverted - the only bear to be found in the entire series is on a secret alternate path in Mission 3 of ''Metal Slug 4'' (a snowy mountain)...and ''he helps you'', attacking the intruding Yeti-like monsters that infest this route. If you shoot him a few times he'll stop helping and leave, though.
** And played straight in that if you stand in the attack hitbox of his claw swipes, you die (with the same animation as though you were knifed).

* BulletHell: An at-the-time unusual example for a series that isn't (usually) a ShootEmUp - several bosses and some of the setpieces will shower you with relentless barrages of bullets. Several bosses are known to fire out geometric bullet "curtains" in concentric waves that would be completely at home in a hectic shmup.

* CaptureAndReplicate: Halfway through the 5th stage in the third game, one of the player characters will get kidnapped by the Mars People. Later, as you storm the Rugname, you'll find clones of said character as your enemy; much later, you'll find the said character in an aquarium-esque confinement where you have to rescue him/her while clones of said character attack you en masse. Afterward, most of the clones turn into significantly more dangerous zombified versions of themselves.

* ContinuingIsPainful: You lose whatever weapon you were carrying, your bombs are reset to 10 (although this is helpful if you have used up your previous supply), and your recaptured hostage tally is reset. The blow is slightly softened by the fact that you will get a free Heavy Machine Gun after continuing.

* CoolPlane: The Slug Flyer is a hilariously small aircraft that functions just as well as a normal one, and comes with a rapid gun and missiles to boot. It can even carry an SV-001 on top of it without any hindrance in performance.

* CutscenePowerToTheMax: Jumping into the Bradley Slug for the first time has the player turn the Bradley Slug around to face the correct direction, something that would be incredibly useful to do in-game but is impossible for the player to do.

** The "wind-up" animations enemy vehicles telegraph their attacks with have been changed and require different timing, presumably to throw off veterans who had enemy behavior hard-coded into their brains across the previous four games.

* DenserAndWackier: The first game only had occasional moments of humor, gave off the atmosphere of a WWII-style {{Dieselpunk}} [[FilmNoir Neo-Noir]] game otherwise ([[TwentyMinutesIntoTheFuture albeit actually being set in 2028]]), and featured an outright depressing credits sequence when finished with only one player. The second leaned hard into the cartoony animations, made the violence [[BloodyHilarious much harder to take seriously]], introduced far more outlandish locations, enemies, and weapons, and the rest is history.
* DependingOnTheArtist: The series has had multiple people providing artwork for it since its inception, and has no consistent style. This has resulted in in-game and promotional artwork run the gamut from being cutesy and {{Animesque}}, to gritty and realistic, to cartoonishly surreal, to [[NightmareFuel outright disturbing]]. Just compare Marco's [[https://static.wikia.nocookie.net/metalslug/images/c/c4/Marco-MS1.jpg/revision/latest?cb=20090919142208 appearance in the first game]] to his appearance in ''[[https://static.wikia.nocookie.net/metalslug/images/7/70/Marco-MSX.jpg/revision/latest?cb=20091006231805 X]]'' or his appearance in ''[[https://static.wikia.nocookie.net/metalslug/images/1/1f/Marco-MS5.jpg/revision/latest?cb=20090921140422 5]]''.

* DevolutionDevice: The scientist enemies in ''4'' fire darts that turn the player into a monkey if they hit, and can also attack with a syringe at melee range that has the same effect. For some reason the transformed player's weapon also changes into an Uzi. If the player dodges the scientists' melee attack, they'll [[HoistByHisOwnPetard stab themselves with their own syringe]] and turn into a different monkey that can be collected for points.

** The first ''Metal Slug'' game has a number of things that seem strange now.

*** The game's tone is fairly grounded, and the humor and cartoony elements made famous in subsequent games is more subdued here. The depressing ending after finishing the game with one player in particular would feel very out of place after the first entry. There are no transformations nor any supernatural elements to speak of.
*** The only weapons besides the pistol are the Heavy Machine Gun, Flame Shot, Rocket Launcher, and Shotgun. The brief diagonal firing that the Heavy Machine Gun does when switching directions is more finicky and unreliable to pull off here. The Flame Shot shoots out a weak fireball that travels far across the screen, as opposed to the shorter-range but incredibly powerful Flame Shot of later entries. The grenades fly at a slightly shorter range and are tinged red. The only usable vehicle is the Metal Slug, although the player can use a turret on the battleship in the final mission.
*** The voice of the announcer is different; he's also present in ''2'' before his better known, [[LargeHam hammier]] replacement debuted in ''X''. The "Mission Complete!" voice clip doesn't play until the "Mission Complete" text has already appeared.

*** You can throw grenades out of the Slug's cockpit while crouching, but you can't collect them -- picking up Bomb crates while riding a Slug always gives you cannon ammo, and you have to get out and pick them up on foot if you want grenades instead. Starting in ''X'', picking up Bomb crates with the Slug in "crouch mode" gives you grenades, allowing you to choose which explosives you get. The health of the Metal Slug itself isn't fixed to a three-hitpoints system. It is possible to take less or more damage than 1 unit of hitpoint depending on the specific attack of the enemy.

*** The Eaca-B and the Flying Tara behave distinctly from one another, the latter will fly from the background first before showing up and drop air-to-ground HE bombs, leave and repeat, while the former will always show up and launch missiles from its bottom hatch before leaving the field for good. In ''2'' and onwards, the Flying Tara behaves identically to the Eaca-B.
*** The jump mechanics of the player character is higher and much floatier and a lot more sensitive (Think ''[[VideoGame/SuperMarioBrosTheLostLevels Luigi]]'' instead of ''[[VideoGame/SuperMarioBros1 Mario]]'').

** ''Metal Slug Advance'' for the UsefulNotes/GameBoyAdvance has Formor, a larger version (2 times) of the Metal Slug.

* {{Flanderization}}: The Bull Chan receive this treatment. In ''1'', they fire from cliffs and only fell down when the edge of the cliff supporting the tank is destroyed, any Bull Chans that show up from the same destroyed cliff will only attempt to plummet down if their target is below them, a similar behavior was adopted in ''X'' when the Keesi acted as a mini-boss, though they may do so at will, either immediately or after firing a few rounds. However, in ''3'' and ''7/XX'', they are only known for their one characteristic, falling and crashing down.

** While the Slugnoid firepower is stronger than the Slugs, it's very cumbersome to use, making it hard to keep it around.
** The Bradley Slug houses one of the most powerful secondary fires available, but due to a bug, is very fragile.

* HarmlessFreezing: Getting hit by Yeti ice breath or blue floating pods will turn the player into a snowman. While this doesn't kill, the player becomes immobile [[SmashingSurvival until they break out of the snow]], and during than they're a sitting duck for enemy attacks.

** For the players, there is the Enemy Chaser, which fires homing missiles that targets the nearest enemies, useful in getting players out of tricky situations.

* InertiaIsACruelMistress: The Slug Mariner uses high-mass rounds, giving it serious difficulty when shooting from below.

* LovableCoward: The SV-Camel refuses to follow the protagonists into an ancient tomb, or into an army of man-eating plants, but is invincible when you do have it.

** Jumping out of any Slug will give you a split second of this, and on top of that will leave the slug undamaged. This is often exploited to great effect in most no-death runs by jumping out of slugs when projectiles would hit them.
** The riot shield soldiers get this once their shields blown away. Too bad they waste it on panicking.

** {{Subverted|Trope}} with the Bradley Slug. It receives no invincibility and can even be hit multiple times with the same bullet.

* MoleMonster: The first level has tiny mole-like robots that periodically lunge out from underground and charge at the player. If they miss, they disappear back underground and try lunging out again and again until they either self-destruct after colliding with and killing the player, or get shot in mid-air.

** In ''Metal Slug 2'', there is much more dakka than the game engine can handle. One thing about ''Metal Slug X'' being an UpdatedRerelease is that the engine doesn't drop frames ''quite'' so much.

** In ''Metal Slug 4'''s final area, if a player gets caught in the explosion, then the ending will show both players hospitalized. Otherwise, they'll show up with the rest eating a large feast on the streets.

* {{Mummy}}: The second level in ''Metal Slug 2''/''X'' is an Egyptian tomb, which has mummies as enemies, which can turn you into one as well with their breath.

** While they still die after one shot, enemies in ''X'' and on don't actually fall over after a single shot and can soak up bullets for a few seconds before keeling over (unless they get knocked over), indirectly nerfing the pistol and machine gun and making weapons that penetrate (such as the laser gun) more valuable.

** Exaggerated in the Xbox (not to be confused with the one on the 360) port of ''3''. Using a continue starts you from the beginning of the level. If you've played the game, you'll know that expecting a player to complete the final mission on a single continue is nothing short of insane. This is particularly grating considering the otherwise identical [=PS2=] version ([[NoExportForYou not released in the US]]) has unlimited continues. It is possible to effectively double your lives on 1-player mode by having two controllers plugged in, and pressing Start on the second immediately after you lose your last life on the first, but that's still a maximum of ten lives.

* RewardingVandalism: If you destroy any objects in the background, chances are it'll drop a useful item.
** Interestingly enough, in one of the levels of ''Metal Slug 2'', vandalism gets punished - if you eat too much food, you'll grow to be fat. '''[[GratuitousEnglish (WHOA-HOH, BIG!)]]''' This means you're slower and a bigger target, so don't lose your tank. Of course, being fat also upgrades all your weapons, so...

** The Elephant Slug can breathe electricity if it eats a car battery.

* SpinToDeflectStuff: Downplayed since they still take damage despite it.
** The Invader drones in ''Metal Slug 6'' use this one quite often. They even roll on the floor and spin-jump around like a [[Franchise/SonicTheHedgehog certain blue hedgehog]]. And they kill you if you touch them while they're spinning.

* StockScream: If the mooks get set on fire or blown up, they'll often let rip one of these. Also, if you die, they laugh and shrug as if to say "what was the big deal?"...only to freak out when you come back thanks to the magic of extra lives.

* VictoryPose: Every character from the main ''Metal Slug'' games does a victory pose in the end of each mission (except in the first game). When playing in co-op, the better performing player gets the honor of doing their pose.

** In general, three lives per continue. The only way to get more lives is to put in more quarters.
** Present in certain ports of the games, each port gives out different sets of continues (some including Free Play as an option).

* WellIntentionedExtremist: General Morden himself for the entire series - trying to take revenge for the Totally-Not-The-UN killing his son, and Oguma in ''Metal Slug 3D''.

* BackwardsFiringGun: The SV-001 Type-R in the first level of the third game has the unique property of firing in the direction opposite the player points. This is important, since the first boss fight is spent frantically running away from said boss.

* BloodyMurder: When you're zombified, your bomb weapon changes into a massive and extremely powerful stream of bloody puke. This attack is so powerful it can destroy tanks in one use, and it even provides the page image for the trope, no less! [[spoiler:It's not as fun when you have to face alien zombie clones of your character, where they ''also'' have this attack]].

* BreakableWeapons: The Slugnoid loses its cannons when damaged. After taking two hits, it's left without any vulcans.
* BulletHell: A rare platformer example of one - several bosses and some of the setpieces will shower you with relentless barrages of bullets. Several bosses are known to fire out geometric bullet "curtains" in concentric waves that would be completely at home in a hectic shmup.

* ColourCodedForYourConvenience: A few examples of this can be found among the series enemies:
** In ''Metal Slug'' and ''Metal Slug 2'', General Morden's infantry includes Sergeant Rank soldiers, who are pretty much like any other soldier except they wear a yellow uniform and they will drop an item for you when killed.

* ContinuingIsPainful: You drop your special gun, and your recaptured hostage tally is removed.

* CowardlyMooks: Barefoot soldiers will often run away scared of you, without even attempting to put a fight.

* DamnYouMuscleMemory: The "charging" animation enemy vehicles telegraph their attacks with has its timing changed up in ''5'', presumably to throw off veterans who had enemy behavior hard-coded into their brains across the previous four games.

** ''Metal Slug 5'' eschews most of the comical parts of the series and plays it almost completely straight-faced. [[BigBad Morden]], [[TheDragon Allen]], and the Rebel Army are PutOnABus and replaced with a much different group of enemies ranging from veiled terrorists to armed mecha. As a result, the game lacks much of the humor found in the previous games, aside from the ability to turn fat. However, most of the new enemies are all edits of Rebel soldier sprites. The game also features a more [[AutobotsRockOut heavy metal-induced soundtrack]] in comparison to the previous games.
** ''Metal Slug 6'' as well, albeit to a much lesser degree. For example, the game features Marco and his team joining forces with both the Rebel Army and the Mars People fighting against a new, dangerous threat that not even the Mars People themselves can handle.

* DenserAndWackier: The first game only had occasional moments of humor, gave off the atmosphere (though not technically the setting) of a WWII-style {{Dieselpunk}} [[FilmNoir Neo-Noir]] game otherwise, and featured an outright depressing credits sequence (at least on 1-player mode). The second introduced far more outlandish locations, BloodierAndGorier enemies and weapons, and the rest is history.

** The fourth game: Trevor's melee attack earns extra points but immobilizes him when he uses it. He's kicking the enemy hard enough to shred them apart. Nadia, on the other hand, whips out a taser (and earns less points than the other characters do with melee, though this is probably a glitch).

*** The grenades fly at a slightly shorter range and are tinged red.
*** Eri and Fio hadn't been introduced yet, and there's no character select screen. Because of this, Tarma can only be controlled by the second player.
*** No transformations.
*** Many staple weapons and vehicles haven't been introduced yet.
*** The Flame Shot shoots out a weak fireball that travels far across the screen, as opposed to the short-range but incredibly powerful Flame Shot of later Metal Slugs.
*** The voice of the announcer is different; he's also present in ''2'' but not in ''X''.
*** The trademark wackiness of the series, though present, is much more subdued than most of the other games.
*** The game is also much kinder and forgiving, and it seems possible to gain a Metal Slug vehicle before each boss fight.
*** Bonus points for the Metal Slug requires the player to bring the tank to the boss arena and keep it intact until the mission ends. Later games instead requires the player to be on a slug during the boss' defeat animation in order to receive credit (doing the final blow on foot does not count).
*** You can throw grenades out of the Slug's cockpit while crouching, but you can't collect them -- picking up Bomb crates while riding a Slug always gives you cannon ammo, and you have to get out and pick them up on foot if you want grenades instead. Starting in ''X'', picking up Bomb crates with the Slug in "crouch mode" gives you grenades, allowing you to choose which explosives you get.
*** The health of the Metal Slug itself isn't fixed to a three-hitpoints system. It is possible to take less or more damage than 1 unit of hitpoint depending on the specific attack of the enemy.
*** The Metal Slug is capable of [[SubsystemDamage actually losing its vulcan gun]] on certain levels without actually losing any health via enemy soldiers climbing aboard and hammering away at the vulcan gun. The second game doesn't have it happen, but the Slugnoid, introduced in the sequel, can lose its vulcan guns.
*** Similarly to the above mentioned point, enemy rebels will attempt to climb and open the hatch of the Metal Slug and throw in a grenade (but the Metal Slug actually takes the hit, and not the player himself).
*** Additionally, enemy rebels will sometimes mount in front of the main cannon to [[TakingTheBullet block the cannon projectile]] from launching, forcing players to [[BulletproofHumanShield spend and waste]] ''[[BulletproofHumanShield three]]'' [[BulletproofHumanShield cannon shots to dislodge said rebel!]]

*** Thematically, the first game is also notable for it's ''lack'' of weirdness. No Mars people, mummies, zombies, lasers, Iron Lizards, or any stuff like that. Just a straight-up war story. It's also less BloodierAndGorier than later games, actually.
** When the Two Machine Guns were first introduced in ''4'', the weapon is treated as a "penetration" type weapon as shown where Rebels died in a rather gruesome manner if used on them. This particular damage-type does not return in ''5'' and onwards, where the the [=2MG=] is treated as a "normal" type weapon instead.

* EpisodeOfTheDead: In the third game, the second level has you fight zombified people, and later, zombified soldiers of the Rebel Army. The reason for the zombie phenomenon is tied to the level's boss: an alien artifact (and the aliens that guard it) mutating civilians.

** During the Iceman route in Mission 2, after going the end area where you can liberate an Elephant Slug to aid you and go back, you will encounter bodies of Yetis who are killed in a surprisingly graphic fashion, courtesy of the zombies suddenly swarming in.
** If a Division 6 fighter pilot rams a wall, their plane gets crushed, along with the pilot, complete with a gruesomely detailed animation of their death.

* GrandFinale: ''3'' was '''intended''' to be this, as SNK was nearing bankruptcy at the time and decided to have one last hurrah. As a result, the craziness of the previous games is turned up to eleven and the final mission is equal parts NostalgiaLevel and MarathonLevel, culminating with a FreeFallFight with the leader of the aliens and ending with the main character throwing their gun as a symbolic gesture that the war is over... [[SeriesFauxnale but then the series continued]] with ''4'' two years later, and currently it's impossible to tell if it will attempt this again.

** Mooks leave a quite respectable trail of blood upon being shot or cut. Shooting down a Mook with the Shotgun results in them literally ''exploding'' into a burst of blood that is twice the size of their regular sprite.

* ImprobableWeaponUser: When in fat mode, your melee weapon (a combat knife or hatchet) is replaced with a really big spork. Or have a seat and whip out your plus-sized belt.

* JetPack: Your character gets one of these in the beginning of 3's final mission, where in case the Slug Flyer / Copter gets destroyed, they will rely on their jetpacks instead. Said jetpack also contains its own built-in rocket launchers in place of the player's grenades. The same jetpack is used during the boss battle of 5, if player's Slug Flyer lose to the Shooting Ray during the aerial combat.

** A trend started from ''5'', later games would have barely if any, character transformations.

** Exaggerated in the Xbox (not to be confused with the one on the 360) port of ''3''. Using a continue starts you from the beginning of the level. If you've played the game, you'll know that expecting a player to complete the final mission with only 5 lives '''at most''' is nothing short of insane. This is particularly grating considering the otherwise identical [=PS2=] version ([[NoExportForYou not released in the US]]) has unlimited continues. [[spoiler:However, in the Xbox version you can effectively double your lives by having two controllers plugged in, and pressing Start on the second immediately after you lose your last life on the first.]]

** ''Metal Slug 4'' seems to have had some screwups in the music department. Early versions of the game had the Final Boss' intro theme switched with his battle theme. The game's OST also had five tracks that are mislabeled and played out of order; the track labeled "Snowy Road" is the most blatant error, as it was clearly meant for Stage 3 yet plays the 1st Boss theme instead.

* PrecisionFStrike: [[https://youtu.be/2W1D-ZotM9E?t=83 It sounds like Tarma is letting one out upon dying].] This can quickly turn into ClusterFBomb given the game's Nintendo Hard nature, which makes his aversion to swears in story-driven installments such as ''3D'' and the tower defense games all the more surprising.

** ''Metal Slug X'' was a redesign of ''Metal Slug 2''. Besides the much-maligned slowdown from ''2'' being virtually absent, ''X'' also introduced the Iron Lizard, Drop Shot, Super Grenade, and Enemy Chaser weapons. It also gave us the BIG versions of the Original weapons (Heavy Machine Gun, Rocket Launcher, Shotgun, Flame Shot, and even the Laser Gun), and increased the difficulty.

** ''Metal Slug XX'' (2009; Playstation Portable, Xbox 360, Playstation 4)

* ''Metal Slug 3D'' (2006; Playstation 2) (Japan Only)

* {{Flanderization}}: The Bull Chan receive this treatment. In ''1'', they fire from cliffs and only fell down when the edge of the cliff supporting the tank is destroyed, and any Bull Chans that show up will fire a few tank shells before eventually crashing it down (the same behavior that they adopt in ''X'' when the Keesi is fought). However, in ''3'' and ''7/XX'', they are only known for their one trait, falling and crashing down.

%% * GettingCrapPastThe Radar: Due to overwhelming and persistent misuse, GCPTR is on-page examples only until 01 June 2021. If you are reading this in the future, please check the trope page to make sure your example fits the current definition.

** The Bradley Slug houses one of the most powerful secondary fires available, but is very fragile.

** Justified in ''Metal Slug 3'''s Mission 1. The water is infested with man-eating fish that will strip a character down to their skeleton should they fall in. Incomparably frustrating if this happens while missing the jump to the Marine Slug (and associated level path).
** Also justified in ''Metal Slug 2/X'''s final mission. The water is ICY COLD and a drop in it will make your character frozen in an ice block.

* ImplacableMan: Allen O'Neal, Morden's bodyguard who soaks more damage than most tanks, and manages to [[BackFromTheDead come back from the dead]] an unnerving amount of times. In ''Metal Slug 2'', he actually gets eaten by a killer whale which even spits out his bones afterward..?and yet, in ''Metal Slug 3'' he's there again, [[GatlingGood gatling]] and idiotic taunts and all, and he amazes us ''even more'' by [[spoiler:[[BackFromTheDead coming back from the dead]] ''again'' and helping you]] in the endgame.
** Although he took a vacation after ''3'', not appearing for a few games (the one in ''Metal Slug 4'' [[spoiler:is actually a robot]]), he returns in ''Metal Slug 7''.

* SamusIsAGirl: As revealed in ''1st and 2nd Mission'', some of the bearded prisoners that you rescue are female, confirming that their general appearance is a disguise. [[spoiler:The second ending for ''1's'' "Another Story" even have a POW shed a disguise to reveal that this particular POW is a female in a bikini.]]

*** There are a number of unique mechanics and interaction with the Metal Slug that's only featured in the first game: Bonus points for the Metal Slug requires the player to bring the tank to the boss arena and keep it intact until the mission ends. Later games instead requires the player to be on a slug during the boss' defeat animation in order to receive credit (doing the final blow on foot does not count). You can throw grenades out of the Slug's cockpit while crouching, but you can't collect them -- picking up Bomb crates while riding a Slug always gives you cannon ammo, and you have to get out and pick them up on foot if you want grenades instead. Starting in ''X'', picking up Bomb crates with the Slug in "crounch mode" gives you grenades, allowing you to choose which explosives you get.

*** The Eaca-B and the Flying Tara behave distinctly from one another, the latter will fly from the background first before showing up and drop air-to-ground HE bombs, leave and repeat, while the former will always show up and launch missiles from its hatch before leaving the field for good. In ''2'' and onwards, the Flying Taras behave identically to the former.

* EarlyInstallmentWeirdness: The first ''Metal Slug'' game has a number of things that seem strange now.
** The grenades fly at a slightly shorter range and are tinged red.
** Eri and Fio hadn't been introduced yet, and there's no character select screen. Because of this, Tarma can only be controlled by the second player.
** No transformations.
** Many staple weapons and vehicles haven't been introduced yet.
** The Flame Shot shoots out a weak fireball that travels far across the screen, as opposed to the short-range but incredibly powerful Flame Shot of later Metal Slugs.
** The voice of the announcer is different; he's also present in ''2'' but not in ''X''.
** The trademark wackiness of the series, though present, is much more subdued than most of the other games.
** The game is also much kinder and forgiving, and it seems possible to gain a Metal Slug vehicle before each boss fight.
** There are a number of unique mechanics and interaction with the Metal Slug that's only featured in the first game:
*** Bonus points for the Metal Slug requires the player to bring the tank to the boss arena and keep it intact until the mission ends. Later games instead requires the player to be on a slug during the boss' defeat animation in order to receive credit (doing the final blow on foot does not count).
*** You can throw grenades out of the Slug's cockpit while crouching, but you can't collect them -- picking up Bomb crates while riding a Slug always gives you cannon ammo, and you have to get out and pick them up on foot if you want grenades instead. Starting in ''X'', picking up Bomb crates with the Slug in "crounch mode" gives you grenades, allowing you to choose which explosives you get.

** The jump mechanics of the player character is higher and much floatier and a lot more sensitive (Think ''[[VideoGame/SuperMarioBrosTheLostLevels Luigi]]'' instead of ''[[VideoGame/SuperMarioBros1 Mario]]'').
** Thematically, the first game is also notable for it's ''lack'' of weirdness. No Mars people, mummies, zombies, lasers, Iron Lizards, or any stuff like that. Just a straight-up war story. It's also less BloodierAndGorier than later games, actually.

* FamilyUnfriendlyDeath:
** For a game with a cartoony art style, people die in surprisingly disturbing ways with lots of blood ''everywhere''. The most notable being the "penetration" death animation where a Rebel is mutilated, decapitated, and maimed.

** For a game with a cartoony art style, people die in surprisingly disturbing ways. And there's blood ''everywhere''.
** Though in certain versions of the games you can change the color of the blood to white, which—depending on how childish you are—will either look like water, milk, or [[{{Squick}} semen]].

* TheLoad: The Chinese soldiers in the Beijing mission from ''Metal Slug 6'' do nothing but chat (and spit on the floor), run around aimlessly, and on one occasion, holding a chicken with a meat cleaver on hand, in the '''middle''' of an Invader attack on their capital! Their only purpose is to act as hosts for Flying Parasites, an Invader bioweapon that can insta-kill even the likes of ''[[MadeOfIron Ralf]]''. This goes over to MilitariesAreUseless, as their troops manning [=MV-280C=]'s (the only one's actually doing their job) ''actively'' attempts to gun down the heroes combating the Invaders.

* SpinToDeflectStuff:

*** Semi-subversion; the bullets do deflect off (while not harming you, thankfully), but the drones will still take damage.
** The trooper clones in ''3'''s final level (spaceship section) deflect HMG and Vulcan bullets while jumping.

* UnwillingSuspension: More often than not, you can find prisoners being strung from treetops, high platforms or ledges by their wrists, hanging in midair until you shoot the ropes to release them. Prisoners dropping from higher levels would make a flapping motion with their arms as they fall before landing.

* WalkingShirtlessScene: Allen O'Neal always fights without a shirt.

* WelcomeToCorneria: Allan O'Neil's pre-programmed dialogue while he battles the players, which consists of "Come on boy! Go home to mommy!" in '''EVERY'''. '''SINGLE'''. '''GAME'''. While it makes sense in the first due to Marco and Tarma being the only playable characters, in the sequels if players are assuming the role of a female commando (Eri, Fio or Nadia) Allen will still continually taunt them with a "Come on '''boy'''!"

** The Chinese soldiers in the Beijing mission from ''Metal Slug 6'' are completely invulnerable to any kind of damage. [[TheLoad They do nothing]] but chat (and spit on the floor), run around aimlessly, and on one occasion holding a chicken with a meat cleaver on hand. Occasionally they might get hijacked by alien parasites and attack you, but shooting them enough only kills the parasite controlling them while leaving the soldiers unharmed. Keep in mind those parasites are capable of killing ''you'' in one hit, even if you're playing as Ralf who can take two hits from a cannon.

* LighterAndSofter: The original game had a relatively grounded military aesthetic and plenty of serious moments (such as backgrounds full of dead civilians and the credits roll that shows all the carnage ''you'' caused), with most of the goofier parts skewing towards BlackComedy. ''2'' and ''3'' introduced much more exaggerated enemy designs, a hammier announcer, ScienceFantasy elements, and character transformations, although ''3'' [[UnexpectedlyDarkEpisode slowly subverts this in its final level]].

** [[VideoGame/IkariWarriors Ralf]] is so badass he can take ''two'' hits instead of one.

** The fifth game introduces Slug Gunner, a modified Metal Slug with legs, PileBunker arm and GatlingGood arm that can also deploy its treads to move faster.
*** An incomplete "prototype" version of it appears in the sixth game, with more standard vulcan guns.

** Allen O'Neal, being a sole person with a gun and grenades, just like you.

** While they still die after one shot, enemies in ''Metal Slug 3'' and on don't actually fall over after a single shot and can soak up bullets for a few seconds before keeling over, indirectly nerfing the pistol and machine gun and making weapons that penetrate (such as the laser gun) more valuable.
* NeverGetsFat: Subverted in ''Metal Slug 2'' and onward; eating enough dropped food items will make the player character(s) fat. This slows them down a bit, but also strengthens their firepower.

* NintendoHard: Not surprising, since arcade games need to be able to get your quarters. There is a rule among fans: "When fighting a boss, use all your grenades immediately. You'll die and get a new set in a few seconds." ''Metal Slug 1'' was hard, yet still the easiest of the games. ''3'' and ''4'' are the most difficult, and one could say the purchase of any one of the games for a home console will pay for itself in saved quarters ''very'' quickly.

* ObligatorySwearing: It sounds very much like Tarma is letting out an F-bomb when he gets killed.

** ''Metal Slug 5''. A lot of the things in the game were cut, and most of it was WhatCouldHaveBeen. For example, we have no reason as to why the Ptolemaic Army are our enemies, or why the [[GiantSpaceFleaFromNowhere Avatar of Evil]] is the FinalBoss of the game.

* PrecisionFStrike: [[https://youtu.be/2W1D-ZotM9E?t=83 It sounds like Tarma is letting one out upon dying.]]
** This can quickly turn into ClusterFBomb. [[NintendoHard You know why.]]
** Averted in story-driven installments such as ''3D'' and the tower defense games. Tarma doesn't even say cuss words in-story.

* RedshirtArmy: Whenever you team up with the Rebels to fight either the Martians or the Venusians, they become this.

* SamusIsAGirl: As revealed in ''1st and 2nd Mission'', some of the bearded prisoners that you rescue are female, confirming that their general appearance is a disguise. [[spoiler:The second ending for ''1's'' "Another Story" even have a POW shed a disguise that reveal that this particular POW is a female in a bikini.]]

* SurvivorGuilt: At the ending of the first game, in the second mission, you find what has usually presumed the daughter of a dead enemy soldier. [[https://www.youtube.com/watch?v=ukBzGrsA1Ds Jump at 2:44.]]
** She only appears if you finish in one player mode, where the credits scroll over the destruction you left behind (graves, devastated buildings, wrecked tanks, etc). Complete a co-operative game, and all the enemy forces are seen having a party instead. The outro music also changes appropriately (or inappropriately, depending on your point of view).
*** Though according to [[http://randomhoohaas.flyingomelette.com/msmia/1/ug.html this]], the enemies in the co-op ending [[WhatCouldHaveBeen were to be Regular Army troops using the Rebel Army sprites, albeit white-colored]].

*** Enemy rebel soldiers that were killed via penetration (as opposed to the resulting explosion) of the cannon's projectile being launched through them are literally [[MadeOfPlasticine dismembered]], [[LudicrousGibs maimed]] ''and'' [[OffWithHisHead decapitated]].

* GrandFinale: ''3'' was '''intended''' to be this, as SNK was nearing bankruptcy at the time and decided to have one last hurrah. As a result, the craziness of the previous games is turned UpToEleven and the final mission is equal parts NostalgiaLevel and MarathonLevel, culminating with a FreeFallFight with the leader of the aliens and ending with the main character throwing their gun as a symbolic gesture that the war is over... [[SeriesFauxnale but then the series continued]] with ''4'' two years later, and currently it's impossible to tell if it will attempt this again.

** In ''Metal Slug 2'', there is much more dakka than [[UpToEleven the game engine can handle.]] One thing about ''Metal Slug X'' being an UpdatedRerelease is that the engine doesn't drop frames ''quite'' so much.

** Taken UpToEleven in the Xbox (not to be confused with the one on the 360) port of ''3''. Using a continue starts you from the beginning of the level. If you've played the game, you'll know that expecting a player to complete the final mission with only 5 lives '''at most''' is nothing short of insane. This is particularly grating considering the otherwise identical [=PS2=] version ([[NoExportForYou not released in the US]]) has unlimited continues. [[spoiler:However, in the Xbox version you can effectively double your lives by having two controllers plugged in, and pressing Start on the second immediately after you lose your last life on the first.]]

* ShortRangeShotgun: Taken UpToEleven. The Shotgun's explosive bursts can't even reach the midpoint of the screen [[note]]most of your munitions in this series travel the length of the screen[[/note]], but can scrap enemy tanks in a few shots. The "Big" Shotgun, first seen in ''X'', ''does'' hit half the screen.

* AbsenteeActor:
** The Rebel Army and Morden don't show up in any capacity for ''5''.
** Morden and Allen O'Neil don't show up in ''Advance''. Allen's son Allen Jr. debuts as the main villain but he's just an optional mini-boss. The Cabrakan is the final boss with no visible controller.

* {{Meganekko}}: Fio is the sole playable character with eyeglasses.

* TheNudifier: The "slimy protagonist death" animations. Males get StrippedToTheBone. The girls, on the other hand, get their clothes dissolved as they keel over, with a brief moment of cleavage or panty shot before they just disappear into a green mush.

* TheNudifier: The "slimy protagonist death" animations. Males get StrippedToTheBone. The girls, on the other hand, get their clothes dissolved as they keel over, with a brief moment of cleavage or PantyShot before they just disappear into a green mush.

* YouGottaHaveBlueHair: Leona from the more recent games have blue hair. And then there's pink-haired Nadia.

* TheGroundIsLava: The final boss of ''[=7/XX=]'' is fought in a huge smelting vat mad eout of the caldera of an extinct volcano, while your character jumps from platform to platform. If you touch the molten metal, you burst into flames and die. It has several moves that involve taking away a few platforms, or dragging them into its milling machinery to destroy them--and you.

* TheGroundIsLava: The final boss of ''[=7/XX=]'' is fought in a huge smelting vat, while your character jumps from platform to platform. If you touch the molten metal, you burst into flames and die. It has several moves that involve taking away a few platforms, or dragging them into its milling machinery to destroy them--and you.

** ''2'' introduced supernatural and sci-fi elements, but they weren't that many and it was rather conservative with them [[note]]The mummies are confined to the second mission, the exploding mutants only appear in a single section of the fifth, and the aliens don't show up until the last portion of the final mission, though they have an expanded role in ''X''[[/note]]. ''3'', on the other hand, ramped up these elements considerably: the very first mission pits you against mutated crab monsters, giant insects and flying pirahnas; the following mission is a ZombieApocalypse (with the chance of fighting yetis depending on the path you take), the fourth mission features the mummies from ''2'', {{Man Eating Plant}}s and an underground tunnel filled with giant insects, snails and worms; and lastly, 2/3 of the (extremely long) last mission is spent fighting against aliens.
** Zigzagged as the series progressed: ''4'' focuses much more on human enemies, featuring only yetis on the third mission and mummies and zombies on the fourth (on different branched paths each); ''5'' is the first game since ''2'' not to feature any kind of fantastic or sci-fi element with the excpetion of the FinalBoss. ''6'' brings back the aliens, and introduces a new kind, the Invaders, who become the main antagonists in the second half of the game; ''7/XX'' focuses on human enemies entirely once again, with the twist that they come from the future and thus use futuristic technology.