The 4 ½ friends consist of three boys, one girl, and a dog. Kalle, the founder and leader of the detective office "Kalle and Sia," the twin siblings Steffi and Karsten, Frinte, and their dog, Pentamorfos, get involved in various adventures in their attempt to solve mysteries and strange cases.

Steffi has a new computer and doesn't let anyone touch it. But when she is persuaded to let her friends play a new electronic game on her computer, trouble ensues! Since none of them have money for repairs, the 4 ½ friends are forced to work as Santa's little helpers in a department store. Kalle takes advantage of the opportunity to offer his detective services to the store. When Kalle suspects that the Santa's bellies in the department store are not only filled with presents, a new adventure begins for the 4 ½ friends.