Heiner and his friend Florian go canoeing on the river Mosel. With them is Monika, Heiner's sister, as Heiner's wife hasn't got time to be with them. But as a joke, Heiner passes Monika off as his wife, and while Monika likes the conceit initially, it becomes a problem when she falls in love with Florian who however wants to remain loyal to his friend. Things get complicated when the trio meet Susanne and Tilo, another canoeing couple, and when finally Heiner's wife Nora arrives, too. Soon quite a bit of partner-swapping seems imminent...
