Twelve-year-old Tessa was looking forward to baking cookies, relaxing, and spending time with her best friend Dennis in their Clubhouse, but with the sudden arrival of Katie - her perfect gymnast cousin from the US - Tessa's plans change.

Katie isn't very nice at the best of times, but now has turned downright hostile, and is threatening to ruin everyone's summer.

When they find a buried chest next to the local dolmen (their small Dutch town's only interesting feature) it sends them on a hunt for a stolen treasure across the Netherlands.

Dennis is thrilled they're going on a real-life treasure hunt, even if he is a little disappointed that there are no pirates involved. But Tessa has wanted to be an archaeologist for as long as she can remember, and knows these items of historical value belong back in a museum.

Soon they discover that they are not the only ones who are after the treasure. They have to form a team and work together, because if the treasure falls into the wrong hands, it will disappear forever.