After breaking up with her unfaithful boyfriend, Katharina Reiff urgently needs a change of scenery. Together with her 16-year-old daughter Nele, she travels to her mother Marianne, who lives on a small island in the North Sea. On the way there, Katharina observes a man being followed by two dubious figures. Good-natured as she is, she takes Sebastian, the stranger's name, with her to the island. When Sebastian's shady pursuers turn up a little later, the shrewd islanders join forces to shed light on the matter.

At the end of winter, there is a game of bowls in the north. The islanders are proud of their bowling ball, which Thies' grandfather found 60 years ago. Only Katharina believes the archaeologist Jens Jeschen, who wants to examine the sphere in a laboratory on the mainland and suspects a Viking grave and treasure on the island, that it is a medieval celestial globe.