The cats have finally found a new home. However, the rift between the brothers Wolkenhimmel and Grauer Flug has also divided the group into two hostile factions. A young tomcat stands between the fronts and must face the hardest decision of his life. Wolkenhimmel denies entry to his forest to any cat that is sick or weak. He even disowns his own son Donner, who is growing up under the care of Grauer Flug among the cats in the marsh. Donner is torn between loyalty to Grauer Flug and the longing for his true father. When a devastating catastrophe strikes the forest cats, only Donner can help them.