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.