The 4 1/2 friends consist of three boys, one girl, and a dog. Kalle, the founder and leader of the detective agency "Kalle and Sia," the twin siblings Steffi and Karlsson, Fritz, and their dog Pentamorfos, get involved in various adventures in their attempt to solve mysteries and strange cases.

The 4 1/2 friends are excited to go on a school trip to a real castle. However, on the first night, Kalle discovers a dark figure on the castle wall. The next day, strange things start happening: one by one, their teachers fall victim to a mysterious illness. The group of 4 1/2 friends, real detectives, begin their investigations to solve another mystery.