Mirroring the structure of Season 2, the third season opens with a captivating three-episode flashback. Adapted from the light novel Dazai, Chuuya, Age Fifteen , this arc explores the origins of the legendary "Double Black" duo:
Returning to the present timeline, the Armed Detective Agency (ADA) and the Port Mafia find themselves caught in a deadly trap orchestrated by the mastermind , leader of "Rats in the House of the Dead".
: We see a 15-year-old Osamu Dazai investigating rumors of the former Port Mafia boss's return.