It's summer and best friends Casper and Emma spend the holidays together in grandfather's beach house. The two go swimming, catch fish and build sand castles. What a pleasure. But then they find an old box on the beach with a mysterious rebus in it. The box appears to be from Emma's grandfather, who buried it a long time ago with his best friend Alfred. Grandpa has not spoken to Alfred for 65 years. Casper and Emma should not think about not seeing each other for so long. The discovery of the rebus is the start of a big treasure hunt, in which the friends scour the entire area for clues. And of course Mrs. Konijn and Welpje join us during the search. The treasure hunt ends with a big surprise and Casper and Emma learn something very important: friendship is the greatest treasure of all.