Uncle Elmeri receives a mysterious old trunk from the 1780s. Its contents reveal it once belonged to a pirate called Captain Haddock. Since they also find a treasure map, the trio sets sail to find the sunken schooner Hispaniola.
After spending a week on a treasure hunt, the boys are still excited. Riku can't resist the urge to fool around on his lookout watch and the boat hits the rocks. Ransu is overboard. Elmeri is slightly worried about his careless crew.
Second week at sea and the trio disembark the boat. While Captain Elmeri and Riku set off to find a spring, Ransu tries unsuccessfully to light their campfire. Feeling blue, Ransu retreats to his thoughts and slips from the rocks.
By the third week, much of the initial enthusiasm has faded. Ransu and Riku have proven to be good sailors, but Riku is more motivated by the thought of treasure. In an attempt to secure the map, it flies to the water.
Another week to go before the treasure can be found. After dropping the anchor, Ransu and Riku land on the island to find fresh water. When they return, both the boat Puskii 2 and Uncle Elmeri are nowhere to be seen.
A four-week journey has come to its end and the wreck of the Hispaniola located. Elmeri dives down to the seabed, while the others handle the air pump. After some dramatic events, the treasure more valuable than gold is in their grasp.