Henkka Virtanen finds evidence of a possible crime on his way to school. With his friend J.K. Kivimutka, the boys decide to investigate what has happened, because these criminal cases are the responsibility of the detective agency they have set up. When it turns out that mailboxes in the area have disappeared during the night, the detective bureau has already two crimes to solve. A fun and exciting adventure begins, in which the boys also have to face their own fears.