To win the favor of the beautiful Duchess of Abrantes, descendant of General Junot, Arsène Lupin stole a cannon from Les Invalides. Under the identity of Lord Wellington, he offers her this strange gift, which is also coveted by the famous oil magnate, Mr. Arkadian. At the same time, Professor Da Silva, a Portuguese historian specializing in 16th-century navigators, whose niece is in love with Lupin, mysteriously disappears. This whole strange story masks the search for the wreck of the Santa Isabella, a frigate sunk in 1805 by one of Junot's cannons. Some say that the wreck contains the jewels of the Portuguese royal family who fled their country.