Boat owners usually wait 25 years for a berth on Lake Constance. But is that enough of a motive for murder? The feisty doctor Anna Junker has long been impatiently waiting for the eccentric Markus Schwarz to vacate his spot for her with his ramshackle old yacht, the "Holde Isolde." Her grief is muted when, of all people, her daughter Feli finds the murdered loner in the harbor basin. The longer Nele and her team investigate, the more mysterious the case becomes. Markus Schwarz was living on his yacht under a false identity. Who was he really? Meanwhile, Swiss retiree Urs Egli is snooping around the marina—until something happens to him on the "Holde Isolde." Paul has a feeling that his old buddy, the harbor master Konrad, is keeping something from him. Mechthild and her grandson Niklas have staked out moorings for five yachts in front of their own house on the lake and are bringing in the excavator right away. It's a real gold mine—until disaster strikes.

Businessman Hannes Burchardt is found dead in the cabin of his yacht, the Constanza. Everything points to a carbon monoxide accident. But upon closer examination, it turns out that he was murdered with dry ice. At first, all the evidence points to his wife, Zola, as the culprit—she wanted a divorce and now stands to benefit from her husband's death. When Nele Fehrenbach's team questions Zola, Zola's lover, organic farmer Michael Jungbluth, intervenes. He is convinced that both the murder and the investigation are part of a conspiracy against him and Zola. Hannes Burchardt, he claims, was deeply entangled in the web of local political corruption and had been conducting shady deals with local political heavyweight Götz Eberlein. When the WaPo refuses to back down, a scandal erupts. Jungbluth and Zola manage to hijack a police boat to force Eberlein and his diligent office manager, Garber, to confess. The WaPo must stop the enraged Jungbluth.

When the WaPo finds the body of a man who's been shot on a rowboat, Nele immediately notices that this case is particularly upsetting to Paul. Paul knows the deceased. He's a convicted robber and murderer. Rudi Maschke robbed a bank 15 years ago. The one million euro haul never turned up. A security guard was killed in the process, leaving behind two children. Paul was part of the task force at the time. Did Maschke go to his ex-girlfriend Maria Ansböck after his release to claim his share of the loot? Maria had broken up with Rudi right after the robbery-murder and now runs a café on the island of Reichenau. His former accomplice, Robin Steffens—who served only seven years and has just returned from Switzerland, now penniless—would also have a motive. But they're too late; he, too, is found dead. Suddenly, Paul himself is in danger. Yet Paul offers Niklas, who has to write an essay for class, a template on the topic of courage.