Gotenba Tori and Katanashi Hisame are private detectives who work together and run the detective agency Knockin' on Locked Door. They are partners and, at the same time, rivals. They use different methods to solve cases and also have different personalities. Gotenba Tori is good at solving cases in terms of how the case took place. He always wears a black turtleneck regardless of season, and he doesn't hesitate to express his feelings. Meanwhile, Katanashi Hisame is good at solving cases in terms of why the case took place. He is guided by common sense and always wears a suit with glasses. These two detectives with different personalities take on the most difficult cases and work together to solve them.