In Nazi-occupied Belgium during World War II, a group of Belgian resistance fighters are tasked with stealing a German submarine carrying a valuable cargo of uranium. They face numerous challenges and obstacles along the way, including treacherous enemy soldiers, claustrophobic settings inside the submarine, and their own personal demons. With their lives hanging in the balance, they must complete their mission and return home safely.

After a heavy storm, the grumpy Captain Eksteroog ends up on an uninhabited island. To spare the time, he decides to search for a treasure that his arch-enemy Zilvertand once buried there. Captain Eksteroog soon notices that he is not alone. Reus, a big guy of two meters and five children with their dog Smikkel have also ended up on the island. Captain Eksteroog and Reus capture dog Smikkel and force the children to search for the treasure. Meanwhile, Piet, Berend, Stien and Steven discover an island that is not on any of their maps. They go on an investigation and end up on the same island where Captain Eksteroog's resides! When they find the children there, they decide to help them find the treasure. But what if they find it and Captain Eksteroog still won't let them leave?