Enomoto Kei is a security "otaku" working for a security firm, devoted to improving the securities system on a daily basis. He is not an easy person to familiarize oneself with, always collected and calm, unapproachable, a maniac in Physics, Science, Architecture and profound in other basic theoretical foundation. He is certain and proud of the fact that there is no key which he cannot unlock.

One day, Kei is asked to help reveal a mystery behind a locked room murderer. While he lacks any interest in solving the mystery, he is inevitably intrigued by the term, "Closed Doors" and decides to take on the position. He works side by side with Aoto Junko and Serizawa Gou, who are lawyers working at a major law firm. Junko is pure and forward, acting upon her instincts while Serizawa is a prideful elitist who sees time as money, and therefore, will not take on any job that is unprofitable.

Possessing a vast amount of knowledge and acute insightfulness, Kei is able to unlock the closed doors and unravel the secrets leading to the solution.