In the first series, Cappie and his friends discover a conspiracy when museums are robbed of their most precious paintings.

In the second series, a descendant of Eduard Schimmelpenninck wants to take revenge on the king for what he did to his family 200 years ago.

In the third series, Tom smells a strange scent at the tree house and discovers there is an intruder there. There is somebody hiding in the tree house who is very afraid.