Aboard the sinking Herculania, George Finné, alias Spy 313, entrusts his sister Jeanette with a secret document. Both government officials and enemy spies are desperate to find the document that Jeanette carries, and she is soon lured into a trap. Meanwhile, crew-member Pierre Lafitte and his friend Lucienne Fereoni decide to become adventurers for hire. When they are overheard discussing the missing shipwreck survivor and warned to stay away from the case, they realize they've stumbled into something serious and are more determined than ever to crack the case.