A lifeguard who drowns? In the case of Daniel Seeger, knockout drops rendered the head of the local DLRG station incapacitated. Nele and Paul investigate the deceased's professional circle and come across his assistant, Leon Berg. The drone expert and motorcycle enthusiast is clearly playing a double game. Then there's the combative environmentalist Carl Weyrich, who felt disturbed by beachgoers and provoked by Daniel. A motive for murder? The WaPo team realizes it has to change its approach. Because, little by little, more and more victims of knockout drops are turning up—people who nearly drowned. For Nele, the case takes on a personal dimension. Her son Niklas, who's skipping school and wants to move out, finds himself in mortal danger. And the DLRG team also includes Anna, whom Noah knows from school. She's happy with her colleague Yannik. Meanwhile, Nele's mother Mechthild is working on a plan to keep the family together.

Mechthild Fehrenbach wanted to record a post for her video blog. When, upon closer inspection of the footage, she discovers that someone on board a ship is writing "Help" on the window with lipstick from the inside, she calls her daughter Nele. Nele is away at a training course and doesn't share Mechthild's suspicion that a crime might be behind it. Mechthild remains undeterred, but she has to pull out all the stops to convince the WaPo that she's on a hot lead. After initial checks by the ship's owner, Stephan Hambach, and a visit on board, nothing points to a crime. However, Sina Bredel, who has just been released from prison, has a score to settle with the ship's owner and threatens to destroy someone's life. While Jakob has his hands full trying to prevent the "Miss Marple of Lake Constance" from continuing her investigation, Sina Bredel's ultimatum is heading toward a deadly conclusion.

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.