21-year-old Shinomiya Yuki has no friends and spends his days alone working part-time jobs. One day, when he returns home as usual after a night shift, a male stranger who has been stabbed in the abdomen with a knife is sitting in his room. The man wakes up at the hospital that he has been taken to and identifies himself as Shinomiya Yuki to detectives. Then he points to Yuki and states that he stabbed him. Yuki flees the hospital right away and becomes a fugitive in an instant. At that moment, a mysterious woman called Zoka appears right before Yuki and asks him to help her take revenge on her father’s killer. He decides to collaborate with her. Zoka is a “leech” who goes from one house to another while its occupants are away. The two of them eventually encounter Kara, a legendary leech who lives like an empty shell solely for the purpose of revenge.

High school student Yoshiro Kobayashi (Mahiro Takasugi) is the great-grandchild of the first president of the Boy Detectives Club. He enjoys time with his friends Watari (Gaku Sano) and Saya (Mayu Hotta). One day, the Fiend with Twenty Faces appears in front of Yoshiro. Mysterious cases then takes place in sequence.