After the death of Captain Thémistocle Paparanic, Captain Haddock's old friend, he inherits a ship called the Golden Fleece. Once Tintin and the captain arrive in Istanbul, where the ship is anchored, they meet Mr. Karabine, a businessman who stubbornly insists on buying it even though it is in a dilapidated state.

Professor Calculus's friend develops a blue-skinned orange that can grow on any kind of land and survive harsh weather (in the manner of Lue Gim Gong) and therefore solve world hunger. The Professor and his friends, however, run afoul of gangsters who also covet the fruit. The adventure takes them from their home in Marlinspike Hall (Moulinsart), a fictional mansion that is presumably in Belgium, to Spain, where Calculus and another scientist are kidnapped.