An aimless college student comes upon the aftermath of a murder scene one night, and without thinking, grabs the titular weapon. He takes it home and keeps it safely hidden away, but curiosity gradually begins to consume him.

Toko grew up unloved by her mother Mizuho who continues to adore her deceased son. She lives alone in an apartment the electricity has been cut off and the trash is piling up. One late night, Toko enters a gloomy building in order to escape from a threatening stalker, Tomita. There she finds a handgun. She brings it back home and confirms it is loaded with four bullets. One day, the family living next door who have already been enduring poverty and violence end up murdering a man. Toko helps them bury the man's body, shooting it with the handgun. Toko becomes captivated by the gun itself. She is further entangled in the true circumstances of the incident due to the detective searching for the handgun and a suspicious man Kazunari who knows a secret about the handgun. Toko tries to throw herself into the whirlpool but then the lid is blown off her "past."