Stranded in a parallel world, Kazuki searches for the injured pilot, Yayoi, but finds she’s been taken by UN forces. Returning to his home, he’s shocked to find his parents don’t recognize him and his belongings are gone. Captured by soldiers led by Mitsuki, Kazuki learns he doesn’t exist in this reality. Ken Sanada, a UN Global Defense Force leader, reveals Mitsuki arrived a month earlier and is a core robot pilot fighting the Rara army, led by scientist Hiroshi Rara. As Kazuki grapples with his displacement, he faces the reality of a world at war.

Persuaded by UN Inspector Akane and Ken, Kazuki becomes the Hartzenen’s pilot, astonishing the team since only females can typically operate core robots. Meeting the alien D, Kazuki trains to control the robot alongside Mitsuki and D. His first battle is terrifying, but when his teammates face danger, Kazuki overcomes his fear to protect them, proving his potential. As the UN marvels at his unique ability, Kazuki begins to embrace his role in the ongoing war against the Rara army.

To conceal Kazuki’s identity from the enemy, Ken designs a female uniform for him, as he has no place to stay in this world. The base’s laughter at his disguise embarrasses Kazuki, prompting him to flee. Encountering Yayoi, who returns his jacket from her rescue, Kazuki reluctantly agrees to wear the suit once, with Ken promising a male uniform for future battles. Akane urges him to comply for secrecy, as Kazuki navigates his new life and the challenges of blending in at the UN base.

Posing as Mitsuki Sanada’s brother at school, Kazuki is overwhelmed by male students vying for her attention, leading to tension when he accepts their love letters. Mitsuki, annoyed, assigns him to respond to each one. When D moves into their home, Mitsuki builds a wall to separate her from Kazuki, frustrated by D’s preference for his instructions. A mix-up with Mitsuki’s diary escalates tensions, but a rainy search brings them closer. Yayoi’s arrival as their neighbor complicates their daily routine, testing their group dynamic.

Jealous of Yayoi’s growing presence, Mitsuki schemes with Reika to arrange Yayoi’s marriage to a wealthy businessman. Yayoi resists, but Reika insists on a meeting near a Rara army battle zone. Despite warnings, the meeting proceeds, and Kazuki’s battle inadvertently disrupts it, delighting Yayoi. During the chaos, Kazuki saves Miss Ra, an enemy, from falling, hinting at unexpected connections. Mitsuki’s plan backfires as the team faces the consequences of their actions in the escalating conflict.

Kazuki, Mitsuki, and D impress the UN with their teamwork, but the Rara army deploys the powerful Original robot to capture Hartzenen’s pilot. Overwhelmed, Kazuki is taken into Rara territory after a surrender signal. Behind enemy lines, he struggles to control Hartzenen, damaging the Rara robot. Ken pleads for Kazuki’s release, but Rara refuses, leaving Mitsuki desperate to act. An explosion raises fears for Kazuki’s safety, testing the team’s resolve in the face of danger.

Searching for Kazuki, the UN finds only his damaged robot. Injured and mistaken for a bystander, Kazuki awakens in Rara territory under Miss Ra’s care, who reveals herself as Mitsuki Rara, resembling Mitsuki Sanada. Despite her kindness, Kazuki plans to escape, wary of her mother, Ayuko, who uncovers his EDF allegiance. Mitsuki Rara, learning he’s a pilot, aids his escape, complicating his feelings as he flees back to his allies, determined to return to the fight.

Back with the EDF, Kazuki struggles with thoughts of Mitsuki Rara. Ayuko brainwashes Mitsuki Rara to pilot the powerful HIMC robot, which rejects her due to her feelings for Kazuki. The Rara army attacks EDF headquarters with HIMC and Original robot copies, overwhelming defenses. Kazuki’s pleas to Mitsuki Rara fail, and HIMC destroys Hartzenen. In a desperate move, Kazuki summons Zinv, a hidden robot, to repel the invaders and save Mitsuki Rara, as HIMC escapes.

Mitsuki Rara joins Kazuki, Yayoi, D, and Mitsuki Sanada at home, but Rara demands her return. The UN seizes her, disrupting Kazuki’s plans, though the EDF defies orders to let her stay. With Zinv, Kazuki turns the tide against Rara’s forces, but Ayuko unleashes a new weapon. A blinding flash causes Zinv and Mitsuki Sanada’s robots to vanish, leaving the team reeling as the war escalates and loyalties are tested.

Kazuki wakes in his original world, adjusting to normal life but missing his friends. In this reality, Rara and Ken collaborate on parallel world research. As barriers between worlds weaken, robots appear, wreaking havoc. Determined to end the war and seal the rift, Kazuki and Mitsuki return to the parallel world, only to find Mitsuki was captured by Rara after arriving earlier, setting the stage for a critical confrontation.

The EDF, disavowed by the UN, flees in an artifact vessel as the war worsens. Jealous of Kazuki’s attention toward Mitsuki Rara, Mitsuki Sanada is tricked into piloting the complete HIMC. Ken warns that the parallel worlds are destabilizing, threatening both realities. As the team struggles to regroup, Kazuki faces mounting challenges to rescue his friends and prevent catastrophic consequences in the escalating conflict.

Mitsuki Sanada, piloting Complete HIMC, overpowers Zinv, trapping Kazuki. D sacrifices herself to destroy Rara’s headquarters, unlocking Zinv’s full potential. With newfound power, Zinv defeats HIMC and saves Mitsuki Sanada. To save both worlds, Kazuki destroys Zinv, sealing the rift. A new parallel world emerges, blending realities and granting Kazuki and Mitsuki Sanada’s wish to stay together with their friends.

In the merged world, everyone adapts to new histories. Ayuko schemes to revive the Rara army and pushes Kazuki and Mitsuki Rara toward marriage. As artifacts are uncovered on the new Earth, plans form to explore space. Kazuki, chosen as Earth’s representative, prepares to venture into the cosmos with Mitsuki Sanada, Mitsuki Rara, Yayoi, and D as his crew, as a revived Zinv awakens to join him.