In The Black Box, Fantasio opened the mysterious box that Jefferson, Boris and Karl left them (see La ceinture du grand froid), and he discovers there the plans of thousands of inventions. When he tests one in company of Spirou, it draws the attention of the commander Alexander, who removes it with Kalloway. Spirou is obliged to yield to them the block box in exchange of Fantasio. Thanks to Jefferson, they discover that the gangsters hid in a stronghold of the Sahara, where they are protected by natives. They go there and succeed in taking again the box to them, while being made pass for deaths.