Henkka Virtanen stumbles upon a piece of evidence of a possible crime on his way to school. With the help of his friend J.K. Kivimutka, they decide to find out what has happened as solving crimes is the mission of their detective agency. When it comes to light that the mailboxes in the area disappeared the previous night, the detective agents have not one, but two crimes to solve. Adventure begins, in which the boys must face their own fears.