Monster X sinks to the bottom of the lake he was blasted into and goes limp. As Enjin rampages above ground however, Aria's necklace begins to spark and shoot energy into X. He draws strength from the necklace as Aria's song plays through his head. He hears Enjin roaring in the distance and grabs the necklace, deciding he's not finished yet.
Enjin ensnares the Siren's van with tentacles from his chest and starts reeling it in. Adagio spots Aria's silhouette inside Enjin's core, struggling to break free. Adagio tries to pull Sonata out of the van, but she's having a panic attack and doesn't respond to her. The van gets lifted too high to jump down from and Adagio gives up, putting her arms around Sonata and musing that at least they'll be reunited with Aria soon.
However as Adagio feels them draw closer to Enjin, something explodes out of the lake behind the Aspect and interrupts it. Monster X, fully formed but only about 50 meters tall due to the divide in power, attacks and grabs onto Enjin, making him drop the van. X catches it with his gravity control and returns it to the road. X briefly grapples with Enjin before throwing him across the forest. Adagio gawks at X's form, seeing it as just another dangerous monster, but Sonata notes his familiar coloring and the fact he's wearing Aria's necklace, except scaled to fit his enormous body, and recognizes him. Adagio and Sonata hold up their necklaces and call out to X, telling him he can have them if he saves Aria. He doesn't agree or disagree, but motions for them to leave to get out of the crossfire. They listen and depart. Enjin comes back and the two kaiju clash.
As the sun sets, Sunset Shimmer drives down the highway with Irys, Megalon, and Gigan in tow. She thinks this adventure is so bizarre, especially after seeing Gigan's mechanical parts which are more advanced than anything this human world can offer and creep her out. Even her former life as a Unicorn in Equestria couldn't have fully prepared her for this. As Gigan and Megalon were regularly transported from planet to planet by Nebulan spaceships, they don't find the car ride anything special, but Irys starts having a panic attack from claustrophobia and the unfamiliar sensations and keeps clinging to the back of Sunset's chair. Gigan checks a map and says that based on what they could see of X through the portal, Everfree National Park is his most likely location. At that moment, the Kaiju sense X and Enjin's energy coming from the direction of the park and tell Sunset, so she drives in that direction.
X blasts Enjin several times, but Enjin shifts into his speed form and blitzes him, launching several strikes that crack X's armor. He then tackles X and knocks him into a rock wall. Enjin charges, intending to impale X on his spiked knuckles, but at that moment, the trapped Aria unleashes a surge of energy. It's taxing, as the core is steadily leeching her power, but it causes Enjin to lose his balance and miss. At the same time, a stream of energy flies into X and strengthens him. He concentrates his power into his fist and punches Enjin's hand, making it explode. He blasts the other hand off, then attacks him while he is vulnerable. Enjin's hands regenerate, sans the spines, and he counterattacks. He grabs X's head and bashes it into the rock wall several times, destroying parts of his helmet and doing serious damage to his head. Enjin tries to blast his head off, but X knocks his hand away in time and the blast hits the wall, creating an explosion that knocks Enjin back. X slumps against the wall under haggard breath, his hearing blown out and his vision blackening in and out every time he took air in past his bleeding jaws.
Kaizer Ghidorah's voice echoes in X's mind, berating him for being weak. As Enjin gets up and approaches, X has a vision of Grand King Ghidorah's attack on Planet Xilian all those years ago, as Kaizer angrily calls X a fake for not having their memories and says he's going to get them killed. Kaizer rips into X's mind and attempts to hijack control now that Aria isn't there to suppress him. X screams in pain as his gravity control starts thrashing the surroundings, making even Enjin back up.
Inside Enjin's core, Aria can tell what is happening to X because he is wearing her necklace. Even with the core draining her energy, she is determined to help him. She starts focusing on all her bad memories, trying to gather enough hate energy to break free. She remembers being chased away by the mermare pod, getting booed off the stage in Canterlot, getting thrown through the portal and waking up as a human, her horror when she discovered she could no longer breathe water, her anger at Adagio for getting them in this situation, disdain for her cousins upon seeing them become accommodated to this mockery instead of wholly loathing it, the dull, bickering rooms full of hexed humans at the mall whose praise meant nothing, her rage and terror upon her first encounter with a monster in this world, then the second and third encounters, at first struggling to learn her new powers under a stern teacher that repeatedly felt the need to lecture her, her alternatingly loathing her lamented situation during her brief break down last night, and then herself in hindsight for doing that in front of someone, storming out a door to get away from her cousins as a human, and her full flung fury upon returning to the battlefield and seeing Monster X hurt, his blood on her hands.
That last memory gives her a moment of pause, wondering why it was so strong but not knowing why. Confused but still trying to gather up her strength, Aria hits her limit to where the hate energy just isn't enough to grant her the strength she needs. Enjin begins to siphon away her insufficient energy and she backtracks to the memory of Monster X being hurt in hopes it would do something. She clings to it like it was her last chance, trying to figure it out and why it had infuriated her so much. The realization strikes her like lightning. The memory was brimming with hatred, but unlike the others, it was hatred in behalf of someone else other than herself. Selfless rage. Righteous indignation. With the world around her starting to go dark, she busily digs through whole new types of memories. She remembers she and Adagio playing in the kelp beds with an infant Sonata. Curling up and sleeping next to her cousins in a sea cave as a siren. The rush of cheering crowds of adoring ponies in Canterlot after their first big break. The first time she managed to levitate and X letting her enjoy the nostalgia. The calm surge she always felt when X allowed her to sing that weird melody to him. The distant voice of a mother she never saw, feeling her muzzle tucking her daughter and nieces into a reef crevice to evade a storm.
With a jolt, she remembers her mother Hymnia singing the three of them to sleep, the same song she used to pacify Kaizer Ghidorah. It was a song that calls on the opposite of their usual songs of hatred and animosity. She sings it now, drawing on all her positive emotions. She unleashes a massive surge of energy and just before she loses consciousness, she sees her necklace is glowing brightly.
Monster X falls to his knees as he starts losing his mental battle. Kaizer prepares to trap him in the same cage he had been trapped in for over 200 years, roaring that their body belongs to him. Suddenly, Aria arrives, and her presence and singing knocks Kaizer back. While X is happy to see her, Kaizer is not. A few days ago, he stopped attacking her in the tree because he thought she was Controller 011. Now that he knows she is not, he is very angry at her, calling her a fake as well and knocking her back. X is assaulted by more nightmarish visions and starts to falter, but Aria reassures him and he remembers her telling him, "Don't lose." With renewed determination, he fights back, resolved to pay back his debts, complete his mission, and not let this dragon ruin everything. Together, X and Aria lock Kaizer Ghidorah back in his cage. In desperation, Kaizer points out X wanted to remember his past and his real name, and Kaizer knows it. X declares that if freeing him is the price of getting his memories back, it is a price he is not willing to pay. X says that his life now, with Gigan, Megalon, and the others, is what matters, not the past. His name is Monster X, and he won't let Kaizer Ghidorah hurt anyone ever again.
As the cage slams shut and sinks into the abyss, X sees a vision of Praetorian Guard 094 and Controller 011 pressing their foreheads together in a private moment. In Xilian culture, this is the closest equivalent it has to a kiss. X at once understands why Kaizer Ghidorah hates him so much, for forgetting the only being he ever cared for. The cage disappears from sight, as X says he is sorry.
Back in the real world, the energy surge Aria released made Enjin thrash in agony as he felt like he was being torn apart. X renews his attack, his strength revitalized. He finds that with Aria's help, he can now access Kaizer Ghidorah's power without the dragon taking control. His body grows a bit taller and adds several tons of mass, crackling and glowing with newfound energy. He easily overpowers Enjin and starts smacking him around, destroying his arm and blasting chunks out of him. Enjin knocks him away with a blast. X's armor is almost completely shattered, and golden scales start to cover his bleeding and bruised body.
Enjin regrows his arm and shifts into his bulkier strength form, but X notices he touched the ground to do that. He remembers when Aria knocked him into the air and then blasted him apart and realizes his weakness: he needs to be touching the ground to heal. He notes that he's running out of time; Aria is likely going to die soon from the energy drain, which will likely deactivate the necklace and change him back into a human. He must defeat Enjin soon. The two battle. Enjin's Adaptive Ability kicks in, his skin is now so hard that X's punches break his own hands from the excess force he's now using, but the sheer force behind the blows forces Enjin back. Enjin fires a blast that X deflects into the lake, vaporizing it. Enjin fires another blast twice the size of the first.
X calmly stands there, then concentrates his power. Enjin is surprised to see his aura looks like a golden dragon flanked by a violet hippocampus. X notes that Enjin's energy ball is made of mana and plasma, but plasma is still matter and all matter is affected by gravity. He catches the blast with his gravity control, and tosses it back towards Enjin and into the ground. The sphere burrows through the ground and then explodes right under Enjin, knocking him into the air. X charges up all his power into his leg like he did his fist before, but to a much greater degree. He charges forward and leaps up, unleashing a powerful, energized kick that blasts off Enjin's arm and lower body, as well as cracking his core. As the two start to fall, the enraged Enjin grabs X's head with his remaining arm and tries to snap his neck. X easily fights the attempt off and breaks the arm, then plunges his hand into the crack. He retrieves Aria, then tears the core in half. The two halves crumble to ash and Enjin gives one last roar as his body evaporates like smoke. Spent, X only has enough power to slightly slow his fall before he hits the ground back first.
It takes several minutes for X to regain consciousness. His body hurts all over and this area of forest looks like a war zone from the crater he is in. He checks his hand and finds Aria had returned to her normal human form after getting her energy drained, but she is unresponsive. Panicked, he sets her down on the soft snow and removes the necklace. He returns to his human form, but he still has all his injuries, racking him with pain. He staggers over to her, but finds that she's not breathing and has no heartbeat. He remembers how he woke her up back at the hospital. He puts the necklace back around her neck, but it doesn't glow. He grabs it, but no force field stops his hand this time. He tries again and again, but gets the same result. He starts screaming for Aria to wake up, but nothing happens. In frustration, he grabs the necklace and sheds tears. Some red smoke flows from him into the necklace. When he notices it glow again, he tries to grab it and this time, it blocks his hand and recoils into the siren. Aria wakes up, gasping for breath.
She asks if Enjin is dead and he confirms it, then she thanks him for saving her. They banter a bit about how often they saved the other, then Aria slyly notes that since she used energy blasts against Enjin, she won their bet and he has to take his mask off. He flinches, then she with concern says he doesn't have to do it if he doesn't want to. He steels himself and tells her to remove it. She gingerly slides his scarf down and pulls his mask off, exposing his face. His face is covered with both fresh injuries and old scars. She decides she likes his smile, then notes the tears still on his face, but he claims he was crying from his injuries. They use the scarf to clean each other of blood, then they look into each other's eyes. Despite the snow and the chilling breeze, they start to feel hot. X closes his eyes and presses his forehead against hers, but Aria soon after grabs his head and kisses him. It might have been a spur of the moment; but for all the trauma they’d been wrung through, they’d earned this.
As they enter the park, Sunset has to stop her car as the road is littered with fallen boulders. Gigan teleports out of the car and looks around. He can sense X but not precisely where. The group can surmise that X transformed to fight something and then changed back. The fight happened far away from here but these boulders were probably kicked up by stray blasts. The group spots the Siren's van approaching, swerving around boulders. Irys recognizes Adagio and Sonata from Photo Finish's pictures. She angrily fires her Sonic Laser, slicing right through the van's fenders and all four tires, making it fall down and screech to a halt. As the everyone is shocked by what just happened, Irys walks up to the van, able to smell X's scent on it, then tears open the driver's car door and demands to know where X is. Gigan restrains her and asks what has gotten into her.
Adagio asks how she knows X. Irys and Adagio get into a shouting match while Gigan and Sunset try to intervene. Megalon walks to the side, trying to figure out what is going on, when he notices Sonata hiding inside the van. He smiles and waves at her, trying to appear non-threatening, and she gets out and joins him. She asks if they are Kaiju, and he confirms all of them except Sunset are. He asks what she is, and she says she is a Siren. They watch the shouting match, then Megalon suddenly realizes that this is like a crossover story, where two or more groups meet and have a big misunderstanding. Sonata, also a fourth wall breaker, understands what he means. Normally, the different groups end up beating each other to a pulp before the real villain shows up, but she would rather skip it and just have everyone sit down and talk things through like normal people. The two give each other a brief summary of what they know. The shouting match is interrupted as Sonata and Megalon suddenly appear in referee uniforms and blow a whistle. Megalon pulls out signs and draws the Kaiju's human forms, before explaining how they are from another world searching for their missing friend X. Sonata explains how they had to team up with X against Enjin, then Megalon explains how Irys assumed the Sirens kidnapped X after seeing the pictures. The others calm down, but are confused at the two pulling random stuff out of thin air.
Sonata says they last saw X when he was fighting Enjin and points to where he was. Gigan looks that way and sees a signature that resembles a mix of X and Kaizer, plus Aria's and Enjin's, which are unfamiliar to him. When he describes it, Sunset thinks he can scry for magic, which offends him as he is a creature of science. Adagio worriedly asks if there was another signal besides X, and he says there is, meaning Aria is still alive. The group of six squeeze into Sunset's car and drive around the boulders to get to the source of the signal. Sunset asks Megalon and Sonata where they pulled those objects out of, but Gigan and Adagio say at the same time not to question it. The two stare at each other for a second and then awkwardly look away.
Aria helps X walk as the two stagger down the road. Sunset's car approaches and stops, then their respective families pile out to meet them. Adagio tearfully apologizes for everything, but Aria hugs her and says she forgives her. Adagio then frantically checks her for injuries and thinks from her flushed face that she's sick. However, Sonata notes X's uncovered face is also flushed and figures out what happened, shooting Aria a grin. Aria rolls her eyes, but hugs her too. For the first time in a long time, she's glad to be with her family.
Gigan and Irys notice X's injuries and stop Megalon from tackle-hugging him. Megalon gives him a forearm bump instead and then helps support him. Irys asks what happened to him and he explains that the creature he was fighting is now dead. Gigan can sense Kaizer Ghidorah's energy within X. Preparing to draw his blade, he asks him to tell him two things only his best friend would know. X describes how the two of them stopped the radio incident with the Kilaaks and then how on the day they first met, Gigan was so shocked at being able to understand him that he stumbled and almost ruined the parade. "I remember well, Gigan. I am your oldest and best friend, I am X." Gigan is surprised by him calling himself that, then X declares he's decided "X" will be his true name from now on. The two grin and bump forearms; since Gigan doesn't have hands in his true form, this is the closest the two had to a handshake.
Irys walks up to Sunset and apologizes for dragging her into this mess. Sunset says everything seems fine now, but Irys says things get significantly more dangerous when they are not human. Sunset looks around and sees the damage to the forest and rock wall. Irys is still surprised she was willing to believe their story, so Sunset explains that she isn't exactly from around here either and she's seen some crazy stuff as well. Irys thanks her for convincing her to make some friends, as she found three really great ones. She hugs Sunset and makes a clicking noise both Gyaos and bats use as a sign of affection.
X approaches them and asks for Sunset's name, recognizing her from the bus stop. She introduces herself and shakes his hand, then he thanks her for helping his family find him.
Bagan watches these events, annoyed at Enjin's failure. It is not a total loss. He was able to cut off Harmony's access to the worlds and X managed to control Kaizer Ghidorah, preventing him from being a loose cannon. When he wants Kaizer's power, he'll just take it himself. For now, he'll continue to play the part of a benevolent master and keep X and the others as pawns until he is strong enough to no longer need them. He opens a portal to the human realm and rests. He waited for 70,000 years and he can afford to wait a little longer.
The portal opens up near the group and Gigan confirms it leads back to Zenith. Irys is sad that they have to go but Sunset says they will see each other again. Irys warns her to run if any Kaiju who isn't them arrives, as they are not on the same team. Sunset makes her promise to join her in the movie theater the next time they meet, promising there will be lots of food. They hug, then Gigan and Irys step through the portal.
Aria tells X to wait, so he motions for Megalon to go ahead. Aria whispers that it was stupid of him to give her necklace back, now he'll have to come back to take it. X brushes it off with the cover that since the necklace was nearly drained of power, it's useless to him. So he plans to just wait and have another go at it once Aria has charged it back up, though Aria seems to be able to tell this is him just making up an excuse to return. Aria crosses her arms and defensively retorts by stating that claiming a siren's heart won't be so easy, but X can see her hiding a smile when she speaks. The two promise they'll stay alive and both be stronger the next time they meet.
X arrives in Zenith and the others help him back to their chamber. As his injuries heal, the group falls asleep. He has a dream instead of a nightmare this time. For the first time in his life, he has something to look forward to.
Aria walks back to Adagio and Sonata and hugs them. Eventually, they get into Sunset's car and she drives back to town. Sunset doesn't know that they are Sirens, but suspects that they aren't just three normal women caught up the same alien crossfire she had. Celestia's former student knows something strange is going on and resolves to stay vigilant.
Aria falls asleep in the car. Adagio holds her and kisses the top of her head, while Sonata covers her with her coat. The two plan to tell Aria about the source of magic they discovered in Canterlot High, while she plans to tell them about Hymnia's song she rediscovered. Either way, the three Sirens are back in business.
