Finally, the Advent season begins! When Klösschen tries to eat the first piece of chocolate from his Sauerlich Advent calendar, he discovers that it is disgustingly bitter and completely inedible. At first, he thinks it's a prank from the boarding school, but when Tim, Karl, Gaby, and Klösschen soon receive an encrypted message, it becomes clear that a repulsive Christmas elf has sinister plans. TKKG dives into the new case and witnesses an act of wanton destruction. And that's just the beginning! What does the disgusting Christmas elf have in store? And why is he targeting TKKG? Will the four junior detectives be able to solve the case before Christmas Eve?.