Ulf think this is going to be an ordinary summer. Going to his grand parents and swim in the lake and tease his brother. But that's not the case, because this summer he will fall in love with Pia.

In an idyllic Stockholm suburb, 11-year-old Lillebror Ström lives with his mum, dad, sister and grandma. When Alex, his sister, introduces her new boyfriend to the family, only Lillebror and his best pal Jens get suspicious. Is Slim Trim really that nice, and what's the deal with those shady friends of his? Meanwhile Lillebror's daddy Roland has financial troubles. Roland works at the Toy Museum, which is host to a gold exhibition. When the greatest gold heist in national history is exposed, Roland's in trouble, being the only one with access to the secret information about the museum's security system. Lillebror and Jens have to do something.