The new school year has begun and Thomas is spending a lot of time with new friends. This makes Senior feel alone and neglected. But when he finds an old model of a ship, he decides to make up an exciting story to go with it.
When Thomas finds a treasure map created by Senior, he begins searching for it with his friends and causes a media sensation. History teacher Hageman, who has a lot of gambling debts, is also interested in finding the treasure.
Publisher Mulder tries to get information about the treasure map out of the children, so Thomas and his friends decide to teach him a lesson. Meanwhile, three thieves are staking out Senior's house to get their hands on the map.
Knowing the map is a fake, Thomas has arranged to 'steal' it while Senior is out to diner. Unfortunately Hageman beats him to the punch. The children follow him to his accomplices' lair in the forest but soon get caught.