A contest is organized in order to make a mini submarine that can explore by 200 metres in depth. The Count of Champignac manage to make such an engine, but it is the target of many sabotage attempts. Xenophon Hamadryas, the contest organizer, reveals that he need the wreck of one his boats, the Discreet, to found be back. However, Spirou face many risks by exploring it.
Tropes associated with this story include
- Instant Expert: The Count, under the influence of the intelligence-boosting X4 becomes an expert at diving after quickly browsing through a book on the subject.
- Professor Guinea Pig: The Count tests his intelligence enhancing Super Serum on himself, after some experiments on a chicken.
- Under the Sea: Much of the action takes place underwater.
- Underwater Base: The villains's base, located in the shipwreck, and joined with mini submarines.
