Adam Cisowski, a precocious and "devilishly clever" high school pupil (the "satan" of the title) spends a summer holiday at a run-down country house, presided over by "The Professor" - an aristocratic, eccentric historian who is friendly and affable but completely improvident.
Adam energetically takes up the challenge, seeking to help his hosts - and in particular, win the heart of Wanda, the Professor's beautiful niece.
Adam must, moreover, play a dangerous game of cat-and-mouse with a nasty gang of criminals, who also got the scent and who are very determined to lay their own hands on the treasure.
Finally, Adam finds the treasure - not before providing the criminals which he knows to be following him with a red herring and letting them "rob" from him a heavy old cask full of worthless scrap iron.
The true treasure, meanwhile found hidden in a stork's nest, is more lightweight - a small sack full of jewellery which the French officer presumably looted in Russia, and whose worth is more than enough to provide for the Professor and his family.