In the lively, post-war streets of Brussels, Belgium, a young journalist named Tintin takes a leisurely stroll through an outdoor market, accompanied by his loyal dog, Snowy. Their quest for unique treasures is abruptly interrupted when Tintin's attention is captivated by a beautifully crafted miniature model of the Unicorn. Just as he is about to make a purchase, he finds himself in the midst of a heated encounter between Barnaby, an Interpol officer, and Ivan Ivanovitch Sakharine, a ship collector, both eager to acquire the coveted model. However, through a series of unforeseen events, Tintin ends up with the model in his possession.
Later, upon returning to his apartment, Snowy’s playful antics lead to an unfortunate accident that causes the model to shatter, revealing a hidden parchment scroll that slips away from view. Meanwhile, bumbling detectives Thomson and Thompson are in pursuit of Aristides Silk, a notorious thief. Intrigued by the Unicorn's secretive history, Tintin visits the Maritime Library. Upon his return, he discovers his home has been ransacked and the model is missing, suspecting Sakharine may be involved. A visit to Marlinspike Hall only deepens the mystery when he realizes there are two identical models.
The day progresses, and Tintin finds his apartment in complete disarray, but with Snowy leading the way, they uncover the hidden scroll. Just then, Barnaby arrives just in time to face a tragic fate at the hands of mysterious attackers while trying to reclaim the Unicorn. Quick on his feet, Tintin manages to stash the scroll in his wallet, but soon finds it snatched away by Silk.
The following morning, Tintin becomes a captive aboard the SS Karaboudjan, a ship ruled by Sakharine’s men. In this perilous environment, he meets Archibald Haddock, the ship's perpetually inebriated captain, whose amnesia adds an intriguing twist to their journey. In a twist of fate, Haddock’s drunken state inadvertently leads them to a lifeboat, which becomes their unexpected escape route from the clutches of danger.
As their attempts to stage their deaths by commandeering a second lifeboat backfire, Tintin and Haddock find themselves piloting a seaplane towards the Moroccan port of Bagghar. Unfortunately, tragedy strikes when their plane crashes in the desert during a fierce thunderstorm, leaving them stranded atop their overturned lifeboat. Together with Snowy, they must use their ingenuity to survive in the hostile terrain while uncovering the secrets hidden within the Unicorn.
As Haddock reflects on his ancestor, Sir Francis Haddock, a captain of the Unicorn in the 17th century, the narrative shifts back in time. Sir Francis faced a fierce pirate, Red Rackham, and made the bold decision to sink his own ship along with most of its treasure—determined to keep it from falling into enemy hands. The legacy of this decision creates three scrolls, each tied to a model of the Unicorn, key to finding the lost ship and its treasures.
In the present day, Sakharine plots to seize the last scroll from Omar ben Salaad during a diversion at a Bianca Castafiore concert. This leads to a thrilling chase through the bustling streets of Bagghar, with Tintin and Haddock not far behind, aided by the comical Thomson and Thompson. Just as they feel hope slipping away, Haddock encourages Tintin to persevere, and together, they track Sakharine back to Brussels, only to find their trap foiled.
In a tense confrontation, Sakharine draws a pistol in a last-ditch effort to evade capture, threatening to destroy the scrolls if his demands are not met. Yet, in an unexpected turn, Tintin swiftly snatches the scrolls from him. Taking advantage of the chaos, Haddock throws Sakharine into the water, turning him over to Thomson and Thompson for arrest.
With the scrolls secured, Tintin, Haddock, and Snowy embark on a thrilling journey with the ancient maps guiding them. As they approach Marlinspike Hall, Haddock’s eyes fixate on an uncharted island—one that shouldn’t exist. Unable to resist, Haddock presses a secret button on a globe, unveiling remnants of Sir Francis’s treasure and a pivotal clue pointing to the Unicorn's ultimate resting place. The film concludes with the duo united in their adventurous spirit, ready to plunge into the unknown, eager to uncover the treasures that await beneath the waves.
