Tintin and Captain Haddock set the trip aboard the Sirius in search of Red Rackham's treasure. Before sailing to Professor Sylvester know Calculus, that shows a submarine that supposedly would help them on their journey. Calculus's deafness prevents him from understanding until Haddock writes that his invention is not welcome.
The ship sets sail, with the company of police officers Thomson and Thompson, as a brother Bird (enemies of Tintin in the first book in this series) has escaped and wandered around the ship before it left port. Later you will discover a stowaway on the ship, Professor Calculus, who took ship secretly with his invention.
The journey continues and after a few mistakes come to the island where she sank the Unicorn and where the treasure is supposed to be. Professor Calculus find a cross on the island, however, after finding no treasure dig. Nor are underwater, so give up and return home disappointed.
When Tintin and Haddock and gave up all for lost, Professor Calculus is a manuscript papers found in a chest of the Unicorn, which identify Hadoque Francis, ancestor of Captain Haddock, as the owner of Castle Mill, who purchase for Calculus captain in gratitude for their help with his boat. When you go there, dedicated to exploring the castle, until they reach the basement where he had been kidnapped for Tintin The Secret of the Unicorn. It was found a statue of St. John the Evangelist, which indicates where Tintin is really the treasure it was, as the scrolls that gave the players the tracks on the situation of the Unicorn, "under the Cross of the Eagle "Tintin and makes the connection between this track and the fact that St. John is called" the eagle of Patmos ". When you remove a picture that hides part of the statue, found a balloon in which marked the island they were before: it turns out to be the key to a spring that opens the globe and reveals its contents, Rackham's treasure Rojo.