In The Secret Of The Unicorn, Tintin buys a model ship in an old market as a gift for Captain Haddock. It turns out that the model ship contains clues about the treasure of the pirate chief Red Rackham. The model ship had been created by the Captain’s ancestor, Sir Francis Haddock, after he was rescued from a deserted island. Sir Francis had killed the pirate who had taken over his ship, the Unicorn, which matches the design of the model ship.
An accident causes Tintin to discover the secret, which is a map to Red Rackham’s treasure. He also realizes that the clues have been distributed among the three models of the ship, and he plans to recover the other two models to continue his hunt. Tintin and Haddock then plan an expedition to recover the treasure.
The Adventures of Tintin: Red Rackham's Treasure continues the story from this point. In this book, Professor Calculus introduces Tintin and Haddock to his new invention, a submersible vehicle that they could use to hunt for the treasure.
As the demonstration does not go off well, Tintin and the Captain tell the professor not to bother them again. But the professor does not hear them properly and builds a new model and stows himself and his machine aboard their ship.
As they set off, the detectives Thomson and Thompson arrive to escort them and to protect them against other possible treasure hunters, as the reason for their expedition has become public news. Now, Tintin and Haddock have to contend with the detectives and the stowaway professor in addition to the problem of finding the treasure.
The Adventures of Tintin: Red Rackham's Treasure takes them to Sir Francis’s island, inhabited by parrots that have a colorful nautical language.
The professor’s invention actually works and helps them explore the depths with ease, but the treasure does not prove so easy to find. Could it all be just a legend? Or could it be possible that they are missing a vital point and should be looking elsewhere (as the professor keeps insisting), closer to home?