In Harmony's realm, she looks through one of her many viewing portals and sees Enjin approaching Monster X and Aria Blaze. She attempts to warn them, but her voice doesn't reach them and the portal gets partially blocked by darkness. Another portal opens and Bagan steps through. His body is shrouded by darkness and the only clear details she can see are his multiple glowing eyes. He explains that he long suspected that some higher being was giving the heroes aid. To rectify this, he sent Enjin to the human realm, gambling that this would cause enough of a disturbance that it would hold her attention long enough that he would be able to detect and locate her. She asks if he was the one who brought the Kaiju to Equestria. He replies that the humans of Terra did that, and once he returns there, they will pay. Harmony senses that Bagan has over four times as much mana as Mothra Lea, and Enjin's power is only a small fraction of it. Bagan says he will not return to Terra until he's "freed" all the Equestrians' souls from their bodies. Furious, she blasts him, making him stagger back, but he weathers it and charges at her.
As Enjin advances on the two, X orders Aria to run, but she refuses. They dodge a punch from Enjin that shatters a tree. The two manage to knock Enjin back, but X explains that Aria's training is not complete, and as they are now, they don't stand a chance at defeating their opponent. X tells her he will hold Enjin off to buy time for her to get to the hotel and warn the others so they can escape, then he will rejoin them as soon as he has an opportunity. She warns him not to die and runs. Enjin tries to follow her, but X kicks him into the dirt. Aria reaches the parking lot and calls Adagio's cell phone, but gets her voice mail. Frustrated, she leaves a message, then hears explosions in the distance. She realizes X isn't just doing a quick skirmish and is fighting Enjin with everything he's got. Angry that he lied to her, she crushes her phone in her hand and runs back.
Harmony is rather scuffed up with her hair undone as she continues to battle Bagan. She makes several tree roots burst out and ensnare him, but he shatters them, then triggers a shockwave that blows up everything within a mile radius. She repairs some of the damage with magic, then ignites a massive Laser Blade from her horn and swings it at him. He blocks it, then creates a miniature black hole that starts sucking everything up. She counters by surrounding Bagan and the black hole with a force field, forcing him to deactivate it to avoid getting sucked in himself. He hits her with a Beam Spam, but she shoulders through it and wraps him in a chain before sending him tumbling down the abyss.
As she stops to catch her breath, she muses that the chain is designed so that the eviler you are, the harder it is to break free. She looks up and sees that some of her viewing portals absorbed some of Bagan's magic and are now playing his memories. She observes scenes from eons ago, back when he was a heroic Kaiju known as Reijuu. He defended the humans and taught them magic, helped raise and train several Mothras and Battras, and helped Mothra and Battra drive off Grand King Ghidorah and his brother DesGhidorah. Depressed, Harmony wonders what could have happened to change Rejiuu to Bagan.
Suddenly, Bagan reveals he has the ability to fly and returns, then shatters the chain. He roars and the portals show his later memories. He turned on the Guardian Beasts and slaughtered the Mothras and Battras until there were only one of each left, then started wiping out the human civilizations, with the panicked citizens begging their hero to tell them what they did wrong. Not so much as a stray plant or bacteria was left. The viewing portals are overloaded and shatter.
He engulfs her with a massive beam and she defends herself with a force field, but the screams of everyone Bagan had ever killed start to invade her mind and the Elements of Harmony exert energy to stop her from being overwhelmed. Bagan muses that Harmony is so strong that Grand King Ghidorah, Mizu, and Doragon wouldn't stand a chance, so he had to take her on himself. As Harmony struggles to withstand the beam, the Elements of Harmony start to crack. Honesty was challenged with the vision of the first attack, the sinking of Thule moments after its monarch had invited Reijuu into the city. Generosity was worked to its limit by the vision of Bagan stepping on a begging, now screaming crowd. Loyalty and Laughter found themselves taxed by Bagan vaporizing a citadel that bore his image in patronage. Kindness found itself splintering as forest and urbanization alike were glassed from the onslaught, billions of lives, human and animal and beyond literally going up in smoke. Even Magic was taking blow after blow as Bagan rampaged across Terra, wiping one nexus of that world's magic after another.
Harmony is forced to her knees, but she looks through the portal Bagan came through and sees Irys, Gigan, and Megalon approaching. She quickly creates a portal leading to Monster X in front of them, which they jump through. Bagan cuts off his beam as he figures out what is happening. Unable to shut off the portal, he instead redirects it so they will arrive several miles away from X. He glares at Harmony, who smirks.
The three Kaiju fall through the portal's tunnel. When they saw it, they assumed Bagan had left it for them and jumped through when they saw Monster X fighting Enjin. When they emerge from the other side, they find themselves in the city, falling out of the sky at an angle. Irys panics and desperately tries to flap her arms and fly, with her wingsuit barely slowing her descent, while Gigan braces himself and Megalon nonchalantly dives down like in a swimming pool.
In her apartment, Sunset Shimmer steps out of the shower and wraps herself in a towel. She's really worried about Irys as she has no idea what happened to her after she disappeared in their last encounter. She thinks about asking her friends to help find her. Suddenly, Gigan crashes through the bathroom window. Groggy, he gets up and tries to get his bearings, but before he can, Sunset screams and whacks him with a hair dryer, then runs out of the room, grabs her cell phone, then runs to the front door. There, she finds Irys, who had smacked into the door and is trying to recover from the impact. Irys is very happy to see her and is pleased to find Sunset is wearing her old necklace.
Sunset is flabbergasted and asks what is going on, but before Irys can answer, Megalon bursts out of the ground, having crashed through the lawn, and says he is okay. Then Gigan staggers up with his sunglasses cracked. Irys quickly hugs the terrified Sunset and assures her they are in no danger, then sheepishly introduces Sunset to Gigan and Megalon and says they are her friends. For the first time, Sunset sees that Irys has fangs, confirming that she isn't an ordinary human. Gigan says they don't have much time so he will cut to the chase. He removes his sunglasses and Sunset flinches at seeing his mechanical eyes, then he extends his wrist blades. He explains that they are from another world and their teammate is in trouble. Irys apologizes and says she didn't want to drag Sunset into this, but if she can help them that would be great. Sunset sighs and asks for a few minutes to get dressed, then she will help them, but she wants an explanation. As she walks back into the apartment, Megalon says he likes her for being so open minded. Gigan asks if they can trust her. Irys says she is the reason why she decided to become friends with them; of course they can trust her.
As he continues the battle, X notices Enjin is growing sluggish and is not healing as fast as before. This is why he abandoned his original plan to stall until Aria could get away and then join her; he feels he can defeat Enjin here and now. After several barrages of martial arts and energy blasts, Enjin has several holes in him, a severely damaged mask, is missing a leg, and has a mangled arm. X attempts to finish him off, but Enjin puts his hands on the ground and absorbs energy from it, instantly healing all his injuries. Then he adjusts his body to become leaner and sleeker like a mandrill. Shocked, X tries to attack, but Enjin is much faster and agile in this form. X tries to levitate away, but Enjin leaps up and tackles him out of the air. X grabs Enjin's wrists and struggles to stop the knuckle spikes from impaling him. Both start charging up an energy blast, with X aware that he will suffer more from a point-blank explosion.
Before either can fire, Aria arrives and blasts Enjin away, then X blasts him as well. He staggers and she catches him, with him scolding her for coming back. Before she can retort, she finds that he's bleeding from several places. Enraged, she puts X behind her, tears off her hoodie, and faces off against Enjin. Her anger causes her necklace to flood her body with its stored power, and she transforms. Her joints cracked and popped, her skin turning a dark shade of fuchsia with golden scales wisping into existence on her arms, neck, under eyes, brow, and jaw. Her hair lengthened noticeably, sprouting a long ponytail while losing her star ties. Gold completely overtook the former green in her hair, the already dark purple becoming a jet black indigo. Twin, draconic horns with alternating black and gold stripes crowned her scalp. Growing several inches taller, the siren opened her glaring eyes; black sclera ringing blood red irises that in turn circled slitted pupils. Finally, as she levitated into the air, energy collected across her back. The spheres of light condensed and flicked out, a pair of wings and a set of twin tails made of golden energy bursting into existence.
Aria gives a bloodthirsty grin and attacks, with her now strong enough to knock Enjin around. Enjin counterattacks, but she remembers the training X gave her and expertly avoids his strikes while continuing to pound him into the ground. Enjin's mask is shattered, revealing a face similar to a gorilla's. He roars and absorbs more energy from the ground, then fires a continuous barrage of energy grenades. She counters by levitating several rocks and trees and dropping them on him. Enjin takes cover, then leaps up and tries to tackle her out of the air, but X blasts him away in time. Having recovered, he tells her that energy blasts seem to be more effective against Enjin. As she charges up a big blast, X attacks Enjin to distract him, then breaks away just as she fires. The blast takes away Enjin's leg, but he's undaunted and tries to attack again. Aria blasts him again, which wipes out his lower body and right arm. As Enjin sails through the air, Aria catches his wrist and then fires a massive blast point-blank, which obliterates the rest, leaving only his hand, which crumbles away, and his core, a glowing bead the size of a softball. As it falls to the ground, it turns dull and lifeless.
At Canterlot High School, Sonata guards the van while Adagio looks around. She admits to herself that Sonata was right, there is strong Equestrian magic somewhere in this school. She frowns as she remembers the fight she had with Aria earlier that day. She could sense that Aria had somehow become much more powerful than her and that irritates her. As the oldest in their family, she is supposed to always be the leader and the most powerful. She then becomes sad as she remembers that despite how much they argue, she loves her family. She has to, as they are the only three Sirens around. Their mother died giving birth to Sonata, and Aunt Hymnia was killed by a storm.
She overhears Flash Sentry helping Pinkie Pie, Rarity, Rainbow Dash, Applejack, and Fluttershy set up a music studio room so that they can practice for their band. Flash asks if Sunset is okay, as she seemed pretty upset the other day after the Daring Do movie, but Applejack says they should just mind their own business. Adagio is overcome with guilt and shame. Ever since the loss of her mother and aunt, it was her responsibility to look after the others, but now she's driven Aria away. As the group in the room starts playing music, her necklace starts to react, but Sonata runs up carrying Adagio's cell phone, which she had left in the van. Sonata plays the message of Aria's warning that Enjin arrived. Filled with dread, the two bolt back to the van.
Harmony and Bagan's extended battle has left them both injured and exhausted, and her realm in in shambles. Suddenly, he turns around and walks back to the portal leading to Zenith. She asks what he's doing, so he explains that neither of them is in any condition to fight, so he'll withdraw for now. She tells him to wait and asks why he turned from the path of the hero to become a destroyer. Annoyed, he gives his answer:
- Bagan: I was lied to for over four and a half billion years. Not again...the past is dead to me. I will go through with my plans, and nothing in Equestria can halt it. Out of anyone in your realms, you stood the best chance at stopping me. Had you been at full strength you probably could have overpowered me in the state I'm in. But, you divided your power. Gave it to others with elements and chests. My return and purge is inevitable.
Harmony approaches him and offers her hoof, telling him it is not too late to end this and turn back to the light. He puts his hand on her hoof and moves it back to the ground. He declares that he is not evil, but extinction comes to all, it is the only truth. As he moves back to the portal, he explains that although he failed to kill her, he achieved his other two objectives. First, he was able to confirm her existence. Second, the battle has left her realm in tatters. Harmony gasps as she sees that in Equestria, the Tree of Harmony has become severely damaged, and the Elements of Harmony are all cracked and chipped. Only the box remains undamaged. Bagan boasts that he lost the battle but he has won the war, because until the Tree and Elements repair themselves, Harmony won't be able to communicate with anyone, so she cannot advise or warn anyone about his actions until it is too late. Harmony sees in one of her viewing portals Aria standing triumphantly over Enjin's core and comments that Bagan won't get a second victory. Angered, Bagan fires a stream of mana at the portal. Harmony blasts him back, but it is too late. Bagan returns to Zenith, promising to kill her last.
Aria goes berserk and blasts the core over and over, but it won't break. X tells her to calm down because they've won and Enjin is dead. She manages to calm down, then marvels at her transformation, happy that she has become so powerful. Suddenly, X reminds her that with Enjin dead, their alliance is over. They face off, but an upset Aria tells him they don't have to do this. X says she wants to protect her family and he wants to obey his Master, so they have no choice but to fight. The two slowly inch toward each other, but find that they can't bring themselves to hurt each other and power down.
She reaches for his face, but Bagan's mana stream enters Enjin's core and revives it. The core expands to the size of a car and sprouts several tentacles. X tries to knock Aria out of the way, but the tentacles are too fast and ensnare her, draining her energy while pulling her towards it. She tries to fight back with energy blasts, but the core and tentacles withstand them. X grabs her and desperately tries to stop her from being pulled in, firing blasts with his free hand. Again, they are ineffective, and despite their combined strength, she's being pulled closer and closer. The core opens up, looking like a mouth. Realizing what is going to happen, Aria makes her choice. She grabs X's face to make him stop firing and look at her, then removes her necklace and places it around his neck. She mutters, "Don't lose." and then shoves him away. She is pulled in and swallowed up by the core. The core levitates and unleashes a massive wave of energy, knocking X into a frozen lake, then it starts absorbing more energy from the ground.
Adagio and Sonata stop their van on a bridge, then look around, trying to use their necklaces as compasses to find Aria. Enjin approaches, now back in his true form of a monstrous gorilla-like being. However, since he didn't absorb Aria's necklace, he's only about 50 meters tall. Enjin wants more energy, and the other Sirens will do nicely.
Despite Enjin's new form, Adagio recognizes him from the hospital, and she panics when she detects Aria's energy coming from him. She floors it, but Enjin smashes the end of the bridge and Adagio barely stops before they go over the edge. She tries to go the other way, but Enjin smashes that end too, leaving them stranded. Enjin roars, a hellish noise that shatters the van's windows and almost completely fills their minds with fear.
In the frozen lake, a barely conscious Monster X slowly sinks to the bottom. Aria's necklace starts to glow, then he can hear her singing voice inside his head. With renewed strength, he clutches the necklace.
