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."
Noda Ryusuke is a popular scriptwriter specialising in stories about the perfect married couple but is shocked when his wife of 23 years suddenly wants to get a divorce.
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.
Hanpeita Takeichi, a samurai, finds that he has travelled through time to modern-day Japan. He's taken in by an old man who runs a cram school, and begins to work as a teacher there.
Based on a manga written by Tatsuhiko Yamagami, the story is set in a former seaport town Uobuka, where 6 former criminals were sent to live there by the government, with the intention of re-socialising them. Aside from the few who know about the project, the general townsfolk has no idea of the former convicts' identities. Tsukisue is the pleasant and efficient municipal official put in charge of the programme. As he slowly learns about their past, a body is discovered.