Miri, a young girl searching for her father, crosses paths with assassins Kazuki Kurusu and Rei Suwa on Christmas Day. To their surprise, the three become roommates. The story centers on Kazuki, a crime coordinator, and Rei, a skilled assassin who grew up in the trade. Kazuki enjoys indulging in his social status and gambling habits, whereas Rei prefers to spend his time playing video games. However, their lives take an unexpected turn when they are entrusted to care for four-year-old Miri Unasaka, who accidentally witnessed a dangerous encounter at a hotel while searching for her mafia boss father.