On a bustling Christmas Eve in Tokyo, a lost four-year-old named Miri Unasaka stumbles upon a grand hotel in her quest to find her father. Little does she know, her innocent arrival disrupts a meticulous plot to assassinate a notorious mafia boss. In the midst of this dangerous scheme are skilled assassins Kazuki Kurusu and Rei Suwa, who live a life devoid of emotional attachment until Miri's presence alters their course. Initially intent on reuniting Miri with her mother, Kazuki and Rei soon find themselves captivated by the child's joyful spirit and end up embracing the roles of parental figures. Despite the challenges posed by their secretive profession, the two assassins, now affectionately called 'papa' by Miri, choose to defy the odds and offer her a semblance of normalcy, navigating the complexities of parenthood alongside their perilous missions.