Tim, Karl, Klösschen, Gaby, and of course, dog Oskar, spend a summer day at Moorsteiner Lake. On the shore, Karl shows his friends his homemade listening device, which transmits intercepted audio signals directly to his smartphone. He attaches the tiny gadget to Oskar's collar, and Gaby sends her dog through the dense reeds to the nearest swimming cove. This way, TKKG becomes eavesdroppers to a strange dialogue about a possible crime. Shortly after, TKKG discovers a backpack full of money in the reeds - money that none of the bathers want. Not even the one from whom it was obviously stolen. The four detectives are faced with a puzzling case.