When a crime goes unpunished due to lack of evidence or the perpetrator's personal influence, Persona XII, a secret organization, steps in and takes matters into its own hands. Its members—all 12 of them being assassins styled after the constellations of the zodiac—are tasked with carrying out justifiable executions.

"Leo" of Persona XII is just one of Kotarou Daigo's aliases. Going by the name Takashi Bidou, he is known as the number one host at club Overload in Shinjuku's red-light district. In his official occupation, he meets clients from all walks of life. Sometimes, these people happen to be victims of the criminals he is set to dispose of—at other times, they are one and the same.

Outside both his jobs, Kotarou takes comfort in his friendship with the number two host, Toru, frequently seeking food or refuge at the other man's home. Little does he know, however, that Toru hides a secret which may soon necessitate a visit from the killer in a professional capacity.