Miri Unasaka, a four-year-old, arrived in Tokyo on her own on Christmas Eve to search for her father. Drawn by the glittering lights and festive atmosphere, she stumbled upon a hotel where she met a man with a delectable cake. Unbeknownst to her, it was part of a meticulously planned assassination plot against a dangerous mafia boss. Kazuki Kurusu and Rei Suwa were skilled assassins who avoided emotional involvement and lived together to escape their tumultuous past. However, their mission was botched, and they returned home with Miri. While the responsible decision was to reunite Miri with her mother, both Kazuki and Rei were smitten by her contagious laughter and pure-hearted outlook. They took on the roles of "papa" for her. Raising Miri was a more difficult challenge than any of their assignments as she was unpredictable and energetic. Despite the risk to their careers, Kazuki and Rei decided to give Miri a normal childhood despite all odds.