Can you not even enjoy a vacation in peace? Hardly have the Karlssons arrived on Aunt Frida's island when Frida has to leave the children to their own devices once again. She has to go to town to sort out the issue with the coins that the wombats have dug up on the island. Maybe it's a Viking treasure. It's just strange that shortly after, the so-called archaeologists appear—suspicious characters who don't exactly behave like scientists. Are they grave robbers? The children find out for sure when the nasty guys lock them up to escape with the treasure. But the Karlssons are not so easily caught off guard. And Alex devises a clever plan.