A stranger asks Thorsten Falke to meet him. He hopes, he says, that the commissioner will keep a promise that is more than 20 years old and help him out of an emergency. Since the stranger demands a commitment without revealing the further circumstances, Falke refuses. Shortly afterwards the man is pulled dead from the Elbe. It turns out he had been living under a false identity. In order to find out who the man really was, Falke and his colleague Julia Grosz have to delve deep into their own past. And with the man the two ultimately suspect, nothing seems to be as initially assumed.