Pettson and Findus are cleaning out the attic looking for old things they can give Henrik for the flea market. Findus comes across a wonderful ‘ship-in-a-bottle’ that he definitely wants to keep. Henrik arrives to take away Pettson’s unwanted items and in the rush of getting everything ready Pettson accidentally puts the ‘ship-in-a-bottle’ in the box to give Henrik. When Pettson and Findus realise what’s happened they rush out after him but he has already cycled away. They quickly chase after him to get the bottle back.
Henrik has almost reached the top of a big hill when his bicycle chain breaks and he goes flying back down the hill. Pettson and Findus suddenly have to run the other way to get away from him, but poor Gustavsson who was walking his dog is too late. Henrik crashes into him sending the cart of objects flying into the air and crashing down again, including the ‘ship-in-a-bottle’. Findus is very sad when he sees that it’s broken but then notices something that was inside. It’s a treasure map! Pettson remembers drawing it as a young boy when he buried a secret treasure.
Knowing that Gustavsson and Henrik will want to get their hands on the treasure, Pettson and Findus have to be very cautious. They decide to go treasure hunting under the cover of night, and with them goes a ‘deadly cackler’, Prillan.
Everything is going to plan, but their luck doesn’t last. At the last minute, greedy Gustavsson and Henrik arrive to claim the treasure from them. All is not lost though, when they see what’s inside the box they think they’ve been tricked, it’s only rubbish!
They give the treasure back to Pettson and his fellow ‘pirates’. When they open it up they’re amazed at what’s inside – Gustavsson and Henrik just didn’t appreciate its value.