Each summer, Ulf travels to stay with his grandparents on an island in Western Sweden. Usually the holiday follows an uneventfully routine pattern but this is the year that, in a completely unexpected move, Percy comes along too. Percy, Ulf's school friend and "blood brother" has misinterpreted a previous conversation and believes that he has secured an invite to join the family on their annual holiday. What's more, despite being considered as unsuitable by Ulf's parents, he enchants everyone that he meets on the island ranging from the ill-tempered grandfather through to a very stubborn horse who is transformed into a pony by comparison when Percy is in the vicinity. It soon becomes clear that Pia, who Ulf has a crush on, also prefers the company of his friend and this threatens to tear them all apart.