Rumi Natsumi is a high school girl who works a night shift on a secretive hotline, weaving a fragile balance between adolescence and a mysterious personal pursuit. One night she takes a call from Ryo Sugiura, a shy, good-looking aficionado of pulp horror who claims to transform into a destructive monster every time he masturbates. Amused and drawn to his odd confession, she agrees to meet him for a date at a diner, where the terrible truth erupts right before her eyes as he shifts into a monstrous form in public.
Intrigued by the challenge and the strangely tender dynamic between them, she follows him as they escape the din of the city and begin a dangerous experiment: he can suppress the transformation by sheer will, and she decides to try to cure him by exposing him gradually to sexuality—habituating him to its pull and its pressure. Their date stretches into the city’s red-light district, a neon-soaked backdrop for their unconventional plan and the unsettling possibilities it holds.
Unbeknownst to them, they are tailed by Maki Nohara, a tough yakuza woman who witnessed the transformation and shares a passion for pulp horror. Her sister, the twisted Oyuki, a sukeban and a classmate of Rumi, becomes obsessed after seeing a photo and tries to blackmail Rumi into bringing Ryo to her. When Rumi refuses and chooses another date with him, Oyuki orders her yakuza gang to kidnap the couple.
Captured and taken to an abandoned building, Oyuki leaves Rumi to be ravished by her men while she takes her pleasure with Ryo. Meanwhile, Maki, suspicious of Ryo’s nature and wary of her sister’s plans, follows the trail with the help of her henchman, Hayata. Rumi fights back, but the insecure Ryo is overwhelmed as Oyuki’s assault triggers a monstrous escalation—one larger and harder to control than before. Maki arrives and tries to intervene, firing a bazooka at the creature, but the monster regenerates and gives chase up to the roof.
On the rooftop, a tearful Rumi presents a keepsake from their date and rebukes him, and the emotional moment jolts Ryo into finally regaining control. In a revelatory turn, it’s revealed that the monster’s true source is an alien entity that has been possessing him, a life-form collector that fed on absorbed life energy. When the entity lashes out at Rumi, Ryo draws strength from the bond they’ve formed and expels the intruder from his body, sending it spiraling into the night sky.
Later, Maki encounters the now-cured couple—Rumi and the reconciled Ryo—as they walk together, and Oyuki is shown working, at least reluctantly, in Rumi’s company. Yet the calm is short-lived: Maki senses trouble emanating from a nearby building, where Oyuki has begun to transform into a monster herself, hinting at a cycle of danger that isn’t over yet.
