Thomson and Thompson come to the Old Market to investigate a mysterious series of thefts of wallets. There they meet Tintin, who purchased in the market model of a ship as a gift for his friend, Captain Haddock. Shortly after buying the gift, is approached by two individuals who will make offers by the model. The first is a certain Ivan Ivanovitch Sakharine, the second, whose name remains unknown, it may be a Barnaby. Shortly after receiving at home Tintin's visit Sakharine, who is trying to buy the boat. Tintin refuses again. Snowy accidentally knocks the boat and the mast broke, and when Tintin is fixing the damage comes Captain Haddock, who is very surprised to see the ship. Tintin takes her home and shows him a picture that appears in the same boat, called Unicorn, after a portrait of an ancestor of Haddock, Hadoque Knight, who lived at the time of Louis XIV. Tintin returns home and discovers that the ship has disappeared. He goes to investigate a house Sakharine Ivan Ivanovitch, and discovers that he possesses an identical boat model you have removed (not the same as the mast is broken). Upon returning home, it is a mess, a sign that you stole the boat is back in search of something more.
The next day, when putting their house in order, Tintin discovers a strange parchment with a coded message. Follows that was hidden in the neck of the model of the Unicorn, and he heads home of Captain Haddock to make him share his discovery. When it comes, the door is closed and Haddock does not respond to calls of Tintin, who chooses to break down the door, and finds his friend, quite drunk, wearing a hat similar to his ancestor of the table and brandishing a sword . Haddock tells him he has been reading the memoirs of Hadoque knight and begins to narrate (stage and at the same time, sometimes to the serious detriment of furniture), accompanied by several gulps from a bottle of whiskey, the confrontation of his ancestor a famous pirate Red Rackham. Follows the story of his ancestor knew the location of the treasure of the pirate. Furthermore, reports Knight Hadoque figure a kind of testament in which he bequeathed to his sons three models of his ship, the Unicorn, indicating that moving the mast come to know the whole truth.
Tintin follows quickly that the scroll you found in your home is one of three that are hidden in these three models ship, and that together serve to know the place where the treasure. However, when going to teach it to Haddock discovers that it has been stolen, as it has gone the portfolio. Then, Tintin and Haddock decide to visit Sakharine, which has another three models ship Hadoque Knight. They find it vanished, and find that the mast of their reproduction of the Unicorn is empty, someone has taken the appropriate scroll. When he recovers, he tells them that has been visited by a man he recognizes Tintin trying to buy the boat in the Old Market. When they leave there, they are approached by the same man, but was shot from a car and falls gravely wounded. Before losing consciousness, so enigmatic notes about birds.
Thomson and Thompson manage to recover some of the stolen purses, including Tintin, which he retrieved the parchment found in his model of the Unicorn. However, shortly after he is kidnapped by some enigmatic characters that surround it in a sort of crypt, somewhere in the field. Escapes from the crypt and finds a warehouse full of antiques. His captors are actually the Bird brothers, antique, and is in the castle of Moulinsart. Both Bird and his butler, Nestor (who end up becoming the steward of Captain Haddock at the end of the next album), trying to capture Tintin, without success. Tintin manages to call the captain for help, and escapes the castle, closely pursued by his captors and Nestor. After many vicissitudes, the reporter manages to overpower their captors, with the help of Snowy and Captain Haddock, appearing in the last minute. One of the Bird brothers managed to escape.
When the pickpocket is finally stopped by Thomson and Thompson (a kleptomaniac who can not resist the temptation to steal wallets), Tintin is the portfolio of one of the brothers Bird, which manages to gather the three scrolls. Together, these indicate a latitude and longitude coordinates, which allow Haddock and Tintin start the treasure hunt pirate Red Rackham. The action continues on the next album in the series, The Treasure of Rackham the Red.