On the night that Ronja was born, thunder passed over the mountains, yes, it was a night of thunder, so that everything unconnected that was in the Mattisskogen was frightened and crawled away into its holes and hiding places.
One day are Ronja seeing a boy (Birk) of her age on the other side of Helvetesgapet - that divides Mattisborgen in half. And Mattis are furious when finds out that his archenemy Borka has moved into the other half of the cracked castle.
On each side of Helvetesgapet lives the robber Mattis and Borka with their robbers. And their children are seeing each other in secret. Until one day when Ronja comes home from the forest and finds Birk both tied up and bloody.
