Shin Hazama is a third-year high school student in contemporary Japan. His mother died unexpectedly during his childhood under mysterious circumstances, an event that profoundly impacted him, leading to emotional withdrawal and a tendency to close himself off from others. His father, Genji Hazama, was a scientist employed by Izumi Heavy Industries. Genji became emotionally absent, dedicating excessive time to his research, which further strained their relationship. Genji's own sudden and unnatural death intensified Shin's isolation, though he maintained a connection with his childhood friend, Kotori Izumi, whose father headed the company where Genji worked. Kotori consistently supported Shin, attempting to bridge his emotional distance.

Jin Hazama originates from a parallel Japan governed under a repressive feudal system controlled by Princess Kotoko. This world diverged from Shin's reality, maintaining an ironclad social hierarchy. Jin experienced a harsher upbringing, marked by the death of his father under oppressive conditions. This loss fueled his rebellion against the regime. Jin possesses a more decisive and hardened demeanor compared to Shin, driven by a mission to assassinate Kotori in Shin's world. He believes this act will eliminate Princess Kotoko, as the lives of individuals across the two worlds are intrinsically linked—death in one world results in the simultaneous death of their counterpart in the other. This connection explains the unexplained global phenomenon of sudden deaths in Shin's world, including the death of Shin's mother, who was the counterpart to someone killed in Jin's world.

The encounter between Shin and Jin occurs when Jin crosses into Shin's world, accompanied by robotic enforcers called Armatics and a fixer named Riko, who serves Kotoko. Jin's arrival shatters Shin's fragile reality, forcing him into a conflict centered on protecting Kotori from Jin's assassination attempts. Initially passive and overwhelmed, Shin gradually confronts the threat, driven by the need to shield Kotori and understand the truth behind the parallel worlds and his family's tragedies. Jin remains focused on his goal, viewing Kotori's death as a necessary sacrifice to dismantle Kotoko's tyranny. His methods are direct and combat-oriented, contrasting sharply with Shin's initial reluctance.

Kotori's counterpart, Princess Kotoko, rules Jin's world with absolute authority, executing dissenters and collaborating on covert scientific projects, including the deployment of fixers like Miko and Riko to eliminate threats across worlds. Jin's vendetta against her stems from personal loss and the suffering inflicted by her regime. Shin's relationship with Kotori becomes pivotal to his development; her unwavering support and their shared peril compel him to transition from avoidance to proactive defense. This shift is tested during confrontations where Shin must physically intervene against Jin and other attackers.

The convergence of Shin and Jin's paths leads to a climactic confrontation involving Princess Kotoko's broader ambitions. Facing a common enemy, their dynamic evolves from adversarial to reluctantly collaborative. Jin's resolve and firsthand knowledge of Kotoko's cruelty complement Shin's growing determination, culminating in a joint effort to thwart her plans. The resolution sees Shin beginning to overcome his emotional barriers, partly influenced by Jin's experiences and Kotori's persistence. Jin's fate remains tied to the stability between the worlds, with his actions leaving a lasting impact on Shin's outlook.