A hikikomori named Yasuo spends his days obsessively watching and secretly photographing three elementary school girls he calls the Flower Trio, or F3: Yuria, the tsundere leader; Chinatsu, the lively and playful one; and Yoshino, the gentle and airheaded one. His voyeuristic obsession pushes him to break into their school and plant a camera in the girls' changing room. When he retrieves the footage, he expects to see them changing, but instead he captures the three of them violently bullying their classmate Haruka, accidentally killing her. He also witnesses them burying her body in the hills behind the school.

Driven by his own past trauma of being bullied, Yasuo decides to confront the girls and make them face justice. But when he corners them, the girls quickly turn the tables, overpower him, and force him into becoming an accomplice. Tied up and called a pig, Yasuo is dragged to the burial site and made to help move Haruka's body. To prevent evidence from being left in the open, he suggests storing the corpse in his own room, where he hides it in a freezer. The F3 now control him completely, using the knowledge of his voyeurism and thefts to keep him in line.

A dark co-dependency forms as the four are bound together by murder. Yasuo's parents, unaware of the truth, simply see him interacting with younger girls. The trio, who were once admired as perfect angels by everyone else, reveal a ruthless, calculating side. As more lives are lost and the pressure of discovery mounts, the group spirals deeper into crime, eventually setting off on a desperate escape with no turning back. The story follows their increasingly grim journey, where innocence and depravity collide.