Spirou, Fantasio, and the Count of Champignac have decided to enter a competition organised by a rich ship-owner: to design a method for human beings to safely dive to and work at a depth of 200 metres. While the Count's genius is up to the task, increasingly serious acts of sabotage begin to endanger his prototypes - as well as the three friends' lives. Who could have a reason to put a stop to their endeavour - and what is the competition's real goal, anyway?
