Hubert is looking forward to a relaxing afternoon at the outdoor pool with his grandma Cäcilia. However, when they unexpectedly run into grandma Elfriede and her grandson Otto, the afternoon takes a turn that Hubert never anticipated: an old rivalry reignites between the grandmothers, as both were successful divers in their youth. While they are certainly not young anymore, the competitive spirit that Hubert has never seen in his grandma suddenly resurfaces. Hubert and Otto first feel the effects of this during the air mattress regatta, as each grandma wants her own grandson to win. But that's not all: the two old ladies want to prove themselves once more. A diving competition will determine who among them is past their prime.