Satoko Sato is a woman whose life has fallen apart after falling for a marriage scam and being caught embezzling money from a yakuza front company. On her knees at the gang's office, she is offered a chance to clear her debts by accepting a bizarre mission: to become the older sister of Lilja, a mysterious elementary school student whose true identity is unknown.

Lilja is no ordinary child. Known to the residents of the underworld as Deathscythe Cutie, she is the strongest schoolgirl in the underworld, a grim reaper who carries a school bag. The story follows the chaotic daily life of these two mismatched sisters as they navigate their unusual arrangement, blending violent gun action with the innocent imagery of school life. The series spans three volumes and concludes with its nineteenth chapter.