Takuyoshi Masuoka, Shinya Tsukamoto, a freelance cameraman who carries a camera everywhere he goes, becomes fixated on fear after witnessing a frightened man, Kuroki, shove a knife into his eye to end his life. He longs to understand the moment that drives such desperation, and this obsession pulls him into a labyrinthine underground network beneath Tokyo. There, he encounters pale, four-legged humanoids that whimper like dogs, hinting at a hidden world beneath the city’s surface. A homeless man living in the tunnels warns him about the “deros,” a term that will haunt his footsteps as he searches for answers. His journey leads him to an underground mountain range and the ruins of a vanished city, where he discovers a nude young woman chained to a wall, the person he will come to know as ‘F’ Tomomi Miyashita. This enigmatic figure does not eat, drink, or speak, and she has oddly formed toes and sharp teeth, suggesting she is something other than human. Masuoka grows obsessed with understanding her, and he rigs up cameras to watch her from his phone whenever he is away from the apartment, turning his investigation into a grim routine of surveillance and care.
Outside the apartment, a woman in a trench coat lingers in the stairwell, and Masuoka later encounters a menacing man in black who bears down on him after he catches F speaking to someone off camera. Inside, he returns to a scene of tension and mystery: F convulses and he cannot feed her, a clue that twelve seconds of camera footage are missing, and a payphone rings with a cryptic warning that he is now in serious trouble. After being beaten with his own camera by a stranger he had filmed, Masuoka cuts his finger on the broken lens and brings himself home to discover that F survives on blood, lapping at his finger and driving him to cut himself further to feed her. He begins to treat F more like a pet than a person, feeding her animal carcasses to sustain her while his fixation deepens. The motherly figure in the trench coat confronts him in the street, insisting that the girl is her daughter, Fuyumi, and demanding to know where she is. Masuoka denies having a daughter and flees, only to find the lock broken and the apartment ransacked when he returns; F is missing and the fear in the walls seems to tighten its grip.
Masuoka wanders the streets in search of F and meets the man in black again, who expresses disappointment in how Masuoka has handled her, speaking to him telepathically in the same voice that rang out from the payphone. When he returns to the apartment, he finds F back on the floor, her hands stained with blood, and the escalating sense that he has crossed a line neither he nor the world can restore. The mother confronts him once more, and he begins filming the confrontation, stepping into an alley as the woman continues to demand to see her daughter. With a cold resolve, Masuoka stabs the mother to death on camera and drains her blood to feed F, who drinks it from baby bottles with ravenous hunger. He then lures a high school girl to a park at night under the pretense of filming amateur pornography and kills and films her as well. A payphone call later confirms to Masuoka that he is, in fact, earning the regard of the stranger on the line for how he is caring for F, and while filming at the scene of the second murder, he sees the murdered girl standing by the crime scene, staring back at him as her body is carried away.
Masuoka leads F from the apartment and abandons her in a karaoke room before boarding a train with no destination in mind. Weeks pass in a restless drift, and Kuroki reappears in an onsen town to discuss Masuoka’s obsession with fear. The elder man contends that fear is primal human knowledge, buried in the subconscious, and Masuoka returns to Tokyo to live among the city’s homeless in the park where he killed the girl. He admits, to himself, that he murdered his wife and a stranger and treated his daughter like an animal, only to later glimpse a pair of deros on the street and discover a cell phone bearing a photo of his own terror-stricken face. Back in his apartment, his wife’s ghost appears unseen behind him in the elevator, and he finds F weakened on the floor. F finally speaks to him, and Masuoka cuts his own tongue to feed her. F leads him back down into the underworld, where she films him once more, and it seems that he has finally touched the fear he sought—the fear that has always haunted him now frames his every movement as the deros’ hidden realm reveals itself.
