A young lawyer with little experience, Junko Aoto (Erika Toda), is hired as an assistant to the famous lawyer Serizawa Go (Kouichi Sato) at a large law firm. On her first day of work, Junko, who was both nervous and excited, was told to meet with the president of a large trading company and was told to follow his wishes. Junko agreed to the president's private request in a daze, that is, to help investigate the closed-room death case of the president of the New Japan Funeral Company. On Serizawa's side, because of Junko, he was accidentally locked up in the bank's vault. When everyone was in a state of panic, Kei Enomoto (Satoshi Ohno) from the security company completed the impossible task and opened the door of the vault with superb technology. Junko, who was full of curiosity, asked Enomoto, who was interested in closed rooms, to help solve this closed-room death case. So Serizawa, Junko and Enomoto formed a closed-room case trio, and solved various closed-room murders one after another, and the locked rooms were opened one by one. The taciturn Enomoto, who is devoted to studying how to pick locks, has gained the trust and love of Serizawa and Junko in a series of cases. But is there some unspeakable secret behind Enomoto?