The 4½ 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, Pippi, get involved in various adventures in their attempt to solve mysteries and strange cases.

Fritz, a member of the detective agency "Kalle and Sia," is in love with the strict biology teacher, Mrs. Sulte-Stratman. However, she suddenly starts behaving strangely. What could be behind this behavior? And is it a coincidence that there was recently a robbery at their local bank? Could she be involved in this case? When Mrs. Sulte-Stratman disappears, the "Kalle and Sia" agency decides to take action.