Shinichi Izumi [Shota Sometani] is a quiet high school student whose ordinary life is upended when mysterious parasites invade, capable of taking over human brains to feed. When one parasite tries to slip into Shinichi’s brain, it fails to bypass his headphones and instead settles into his right hand, a surprising twist that leaves Shinichi alive and conscious in a body that now hosts two minds. Through this unlikely bargain, Shinichi and the hand-born parasite, Migi [Sadawo Abe], forge a wary yet functional partnership. Shinichi’s humanity clings to life as Migi’s logical, emotion-light perspective begins to temper his own impulses, leading to a delicate balance between two very different ways of thinking living inside one body. The two minds, learning to coexist, carve out a fragile space where empathy still flickers, even as Shinichi’s personal life grows tangled and strained.
Parasites strike with stealth and brutality, feeding on humans while keeping themselves hidden. One parasite in particular emerges from the shadows by infiltrating Shinichi’s teacher, Ryoko Tamiya [Eri Fukatsu], transforming the dynamics of school and home as she pursues a broader curiosity about human life. Tamiya’s unusual viewpoint—curious about people from the inside out—grows into a dangerous dual existence when she becomes inexplicably entwined with another parasite, Mr. A [Mansaku Ikeuchi], who embodies a vision of coexistence that challenges the boundaries between species. The discovery that their offspring, conceived while Tamiya’s parasite is present, can still be human adds a troubling layer of ambiguity to what “life” means in this new ecology.
The conflict intensifies when the parasite Mr. A’s ambitions collide with Shinichi’s world. After a failed attack on Shinichi results in the destruction of his vessel, Mr. A transfers his consciousness into Shinichi’s mother, Nobuko Izumi [Kimiko Yo], a drastic move that plunges Shinichi into a personal nightmare. Nobuko returns home bearing the parasite’s influence, and in a cruel moment Shinichi is mortally wounded by his own mother’s body. Yet Migi’s essence reconstitutes Shinichi’s heart, inserting Migi’s particles into his system and effectively fusing the two beings’ lifeforces. From that point on, Shinichi’s identity grows more stoic and detached, a result of their merged consciousnesses, and his emotional distance strains his relationship with Satomi Murano [Ai Hashimoto], his girlfriend who once inspired warmth and hope.
As the town begins to feel the parasites’ reach more acutely, Takeshi Hirokawa [Kazuki Kitamura]—an ambitious underling of Ryoko Tamiya—campaigns to secure the mayoralty, setting the stage for a future in which the parasites can control the community from political power. The quiet surface of Shinichi’s life is torn further when another parasite, Hideo Shimada [Masahiro Higashide], transfers into Shinichi’s school and hides in plain sight as a student. When a sharp-eyed classmate discovers Shimada’s true nature, the parasite brutally massacres the students before Shinichi can intervene. Shinichi does not falter, eventually managing to kill Shimada, but the predator’s control over the body leaves a lasting coldness behind. Tamiya, with a calculated calm, reveals the location of Mr. A-possessed Nobuko, and Nobuko herself fights long enough to allow Shinichi to end her torment safely.
In the aftermath, the town’s power struggles and the ongoing threat of the parasites continue to unfold. Hirokawa’s political ascent proves to be a real possibility, and the enigmatic parasite Goto [Tadanobu Asano] makes a chilling appearance, hinting at larger, unseen currents guiding humanity’s fate. Shinichi’s personal stakes remain high as he visits Satomi in the hospital, hoping for normalcy while a mysterious observer records the exchange between Shinichi and Migi, capturing the uneasy coexistence that defines their strange partnership. The film closes on a note that blends caution and resilience, underscoring how two worlds—human and parasite—must navigate a shared future in a world where survival hinges on both adaptation and a stubborn glimmer of humanity.
