Yōta is a university student carrying the scars of a childhood spent being bullied. Unaccustomed to forming bonds with others, he views Maiko, an older female senior who acknowledged him for the first time, as something close to a savior. His lonely devotion quickly deepens into romantic longing. But when Maiko finally draws him into her world, the invitation is not what he expected. What she demands of him is an "abnormal sexual act" with her own lover, a woman named Nene. Yōta, desperate for any connection to Maiko, complies.

That single encounter becomes the pivot point of a slow psychological collapse. Ensnared in a triad defined less by love than by manipulation, Yōta finds himself performing intimacy without affection, trapped in a one-sided obsession he cannot break free from. Meanwhile, Nene, who also loves Maiko, begins confiding in Yōta and eventually voices her own growing suspicions about Maiko's true nature. As the two pawns draw closer to one another, the bond that was meant to isolate them instead becomes a possible path to clarity, or deeper ruin.

The story unfolds within a contemporary Japanese university and social setting, its atmosphere thick with emotional dependence, erotic tension, and quiet dread. Maiko functions as the magnetic, destructive center of the narrative, a "femme fatale" whose affections are meted out as tools of control rather than tenderness. Yōta, isolated and vulnerable, becomes her instrument. Nene, at once a rival and a fellow captive, shares his confusion and complicity.

The series traces the accelerating decay of Yōta's ordinary life across a tightly constructed nineteen-chapter arc. Key narrative movements include his initial submission, the gradual shift in his relationship with Nene, and the exposure of the psychological manipulations that bind the three together. The title itself, Parasite of Loneliness, gestures toward the core conflict: love not as connection but as consumption, where human beings swarm around affection the way insects gather to feed.