A jogger bites into dune grass and seemingly dies of natural causes. Did Hardy Lohmann simply overestimate himself during the traditional mile run on Borkum? Commissioner Mona Sander is on-site as a first responder and notices suspicious symptoms. An autopsy of the deceased provides certainty: the victim was poisoned. Did his young, beautiful mistress have a hand in it? And what role does a dubious cease-and-desist lawyer play? Who does the 100,000 euros found in Lohmann's vacation home belong to? Mona Sander and her colleague Enno Moll begin investigating the murder case. But when an opaque man from Mona's past suddenly appears, events take a dramatic turn.