An Italian engineer who had made a strategically important invention to ward off enemy aircraft is killed in a robbery. As the plans have presumably fallen into the hands of spies, secret service captain Bergall is given the task of recovering the papers. Bergall initially adopts a false name and appears as Mr. Gran, whom nobody knows. Under this name, he rents a room in the Hotel Danieli in Venice and makes the acquaintance of the shipowner's daughter Viola Dolleen. With her help, he is able to eliminate Captain Gordon, who is also interested in the plans. When Gran learns that the plans are now in the possession of the art dealer Tschernikoff, who wants to sell them to Gordon, he immediately travels to Rome, visits the art dealer Titian and pretends to be Gordon. In the art dealer's remote villa, events come thick and fast...