On a quiet night in Brussels, a pair of criminals break into an aquarium and steal a priceless pearl. They replace the real gem with a convincing fake and slip away as the guards rush to raise the alarm. The director initially chalks it up to the guards’ story, but a second, similar museum theft from two weeks earlier makes him pause and reconsider what actually happened.
Tintin, Snowy, and Captain Haddock travel to Syldavia, where their friend Professor Calculus has rented a lakeside villa to tinker with a daring invention. At the airport they encounter Thompson and Thomson, and the trio (plus Haddock) join Calculus on a flight to his secluded retreat. A mechanical hiccup forces the pilot to bail out, leaving them with only one parachute. Tintin bravely tries to steer the damaged plane toward safety, but it careens toward a cliff and bursts into flames. They are miraculously saved by two local children, Niko and his sister Nouchka, along with their dog Gustav. The rescue is short-lived, however, as the pilot’s walkie-talkie message to his boss reveals the crash was not an accident but part of a larger plan hinting at a shadowy figure behind the scenes.
The children warn them of a lake-bound curse beneath an ancient, sunken town. The group finally reaches Calculus’ villa, where the scientist unveils a camera capable of projecting holographic images and, in his long-term vision, creating actual copies of physical objects. In the ensuing dinner, Thompson and Thomson explain that Calculus has become the target of an art-forgery syndicate intent on stealing his creation. That night, Snowy’s sharp ears pick up a noise outside, but Tintin initially thinks it’s bird chatter. It turns out Madame Flik, the housekeeper, is in league with the mastermind known as “Mr. Big.”
The next day brings further peril. Tintin explores the countryside with Niko and Nushka while Haddock and the detectives play golf. Snowy encounters a frogman who has obtained stolen plans from Madame Black, and the dog manages to bite off part of the intruder’s flipper. A hidden cable leads Tintin to a buried passage that opens into a cave where the criminals stash stolen art. He is able to escape a net by Snowy’s timely intervention, and later returns home to see Calculus demonstrate a new copy device using a soap-like material. The copies are imperfect and transient, but the criminals attack with laughing gas and kidnap the children. Tintin and Haddock pursue them but cannot rescue Niko and Nushka. The villains leave a taunting message on a tape from their leader, the timbre of whose voice Tintin recognizes, demanding a ransom in exchange for Calculus’ invention and ordering Tintin to make the swap alone.
Tintin, Haddock, Calculus, and the detectives set out to track the criminals. Tintin discovers a secret passage that reveals Madame Black’s walkie-talkie hidden in an abandoned well—an evidence-rich breakthrough that prompts him to contact the police. On his way to town, Tintin is joined by Bianca Castafiore Bianca Castafiore, who gives him a ride and helps him reach the authorities. The local police chief is constrained, however, because part of the lake sits in Borduria’s jurisdiction, preventing a full-scale international response.
Two days later, Tintin returns with Calculus’ shark-like submarine, and the plan unfolds: Tintin will meet the crooks on the beach while Haddock tails him in the submarine. At the rendezvous, Rastapopoulos—the mastermind behind the operation, sometimes called “King Shark”—takes Tintin and the children to the underwater base beneath the flooded city. There, Rastapopoulos unveils his scheme to mass-produce art by using Calculus’ machine to clone masterpieces, a prospect he relishes in flaunting with his hoarded collection.
A trial run using a carved cigar box backfires when the clone grows dangerously large, nearly crushing Rastapopoulos and his lieutenant. Angered, Rastapopoulos locks Tintin and the children in a chamber as his base begins to flood. He orders a hasty exodus, planning a final rush to escape before the self-destruct sequence detonates. Haddock manages to regain control of his disabled submarine and surfaces, aided by Thompson, Thomson, and the Syldavian police. The base’s inhabitants evacuate, but Rastapopoulos and his lieutenant slip away in their submarine.
Tintin and the children wriggle free and escape through an airlock just before the base explodes. They surface to rejoin their friends, and the police round up Rastapopoulos’ gang, while Rastapopoulos himself tries to flee across the border in another submarine. He fails to account for a stalled periscope, which clips a rock and seals his fate; Tintin and Haddock quickly close in and capture him as soon as he attempts to leave the wrecked vessel. With the danger over, Bianca Castafiore returns for a celebratory reception at Calculus’ villa, where villagers, friends, and the reunited crew toast their survival—though Haddock would rather avoid the party’s exuberance, until a playful misfire from the Thompson twins’ golf antics gives him a reason to laugh along with everyone.
