Red Rackham's Treasure

In the previous adventure, The Secret of the Unicorn, Tintin and Captain Haddock discover three parchments revealing the location of the Unicorn, a 17th century navy ship commanded by Haddock's ancestor Sir Francis Haddock. The Unicorn was scuttled by Sir Francis while battling the pirate Red Rackham for his treasure. Tintin and Haddock believe that the pirate's treasure is in the remains of the sunken Unicorn and they hire a fishing trawler, the Sirius, to search for the treasure.