One night, in Brussels, Belgium, a pair of crooks discreetly break into the aquarium and steal a priceless pearl. As soon as the security guards on duty see the empty shell, they rush away to raise the alarm. The crooks take advantage of the guards’ absence by putting a fake pearl, the same size as the real one, in the shell. When the guards return with the director and the director sees the fake pearl, he thinks the guards were making it up, but then has second thoughts about the incident as a similar case happened at another museum two weeks before. The moment one guard says “a real shark,” the guards and director look at the aqaurium’s shark, the only witness to the crime, swimming in its tank. The scene then shifts to the opening credits.
Some time later Tintin, Snowy and Captain Haddock arrive in Syldavia, a country in the Balkans. They have come to join their friend professor Cuthbert Calculus who has rented a villa near a lake in order to build his latest invention. The Captain is especially keen in improving his golf. At the airport they run into Thompson and Thomson who are also heading for Calculus on a special mission.
The four men and dog fly by hired plane to Calculus’ house, but during the flight they get into some engine trouble and the pilot bails out with the only parachute. Tintin attempts to safely land the plane, but it ends up on the edge of a cliff and on fire. They are saved with the help of two local children, Niko and his sister Nushka, and their dog Gustav (who is somewhat goofy and bumbling compared to Snowy). The pilot discreetly watches this from a distance. He then contacts his superior (“Mr. Big”/”Shark King”) via a walkie-talkie disguised as ordinary binoculars, telling him that their plan to kill Tintin and his friends has failed.
The children give their new friends a lift in their wagon. They warn them that there is a curse on the lake, at the bottom of which is an old submerged town. The party finally arrives at Calculus’ villa where he demonstrates his invention: a camera which can project holographic images. It is part of a far more ambitious project: a machine which will make actual copies of physical objects.
Over dinner Thompson and Thomson explain that they are Calculus’ bodyguards as they suspect that a criminal organization specializing in making art forgeries wants to steal his machine. They tell of the thefts from the museums and how the copies have become more and more sophisticated in the past six months.
Eventually, everybody goes to bed, with Haddock taking a bottle of whiskey (which he smuggled into Syldavia, following the events from Destination Moon) out of his golf bag and taking a swig of it, and Thompson throwing an alarm clock at Thomson to stop his snoring. Snowy wakes up Tintin at one point after hearing a noise outside, but Tintin shakes it off as bird calls. Outside, Calculus’ housekeeper, Madame Black, contacts “Mr. Big” by using a walkie-talkie hidden in the dried-up well.
Next morning, while Tintin explores the local country with Niko and Nushka (unaware that there are cameras spying on his every move) and the Captain and the detectives play a game of golf, Snowy runs into a man in scuba gear who has obtained from Madame Black some plans stolen from Calculus’ laboratory. The man escapes by jumping into the lake, but Snowy manages to bite off a part of one of his flippers.
After the Captain and the detectives tell Tintin what has happened, he gives the dogs the bitten-off flipper to sniff. While Gustav leads Haddock to a pile of abandoned tyres, Tintin follows Snowy to a buried chain which, when pulled, opens a passage to a hidden cave where the criminals have stashed the stolen art. Tintin recognizes the cave as the one discovered by Ali Baba. After getting sealed into the cave, Tintin finds an underwater tunnel leading out to the lake. On his way through, Tintin gets trapped by a wire net, but Snowy (who remained outside the cave) dives in and chews through the net, saving Tintin from drowning.
Back at home, Calculus demonstrates his new invention to the children — a machine that can copy any object from a piece of special soap. Unfortunately the effects are short-lived, as the copied objects (in this case Thompson and Thomson’s hats) turn back into soap very fast. The criminals attack Calculus and the detectives with laughing gas and take the children away. Tintin and Haddock pursue them but fail to rescue Niko and Nushka, when one of the henchmen fires a tommy gun at them. The criminals leave behind a message on a tape player from their leader, “King Shark”, who tells the heroes that they will get the children back in return for Calculus’ invention. Tintin himself is to do the exchange and is not to call the police. The reporter realises he has heard the voice from the tape somewhere before.
Tintin, Haddock, Calculus and the detectives discuss the situation. They search the house for bugs and Tintin discovers a secret passage that leads to the walkie-talkie hidden in the empty well. He then catches Madam Black in the process of obtaining it. Haddock tries to make her talk, but Tintin decides that she is just a pawn and doesn’t even know who “Mr. Big” is. At the same time, the children are taken to “Mr. Big” and he orders his henchmen to watch the house.
Tintin decides to contact the police and comes up with a ruse to cover his tracks. From a nearby hide-out criminals are watching the villa and report to their boss that Tintin and Haddock are simply pacing up and down the room: in fact they are dummies being pushed around by Thomson and Thompson who are on their knees in order to avoid being seen.
Meanwhile Tintin and Snowy set off to the local town. Tintin comes to the home of a man named Vladislav, but, instead of letting Tintin use the phone, he shoots at him with his blunderbuss, thinking that he is a prowler. Vladislav tries to call the police himself but one of “Mr. Big’s” henchmen cuts all the phone-lines in the area.
Going on his way Tintin runs into his friend Bianca Castafiore, the opera singer, who gives him a lift in her car to the town. Tintin notices two criminals near the police station (including the pilot who had tried to kill him earlier), but Bianca tricks them by sending her assistant Igor Wagner with Snowy first, thus making them think it’s Tintin. The criminals attack Igor and Bianca runs to him screaming in panic. Since this alarms the police, the crooks run away.
While Bianca starts singing in order to prove to the policeman who she is, Tintin sneaks into the station in disguise. The chief of police listens to his story, but he is limited in what he can do since half the lake is in the jurisdiction of Borduria, a rival nation, and there are thus risks of a diplomatic incident.
Two days pass and Haddock and the others have had no word from Tintin. He finally arrives with the shark-like submarine which Calculus built during the search for Red Rackham’s Treasure. The plan is simple: Tintin will meet the crooks on the beach, and Haddock will follow him in the sub.
Criminals pick Tintin up in their submarine and they head underwater to a flooded city where their base is situated. The submarine docks in an old cathedral, where the villains have set up their base. Tintin’s arch nemesis Rastapopoulos (who’s more of a James Bond-type villain in this movie) turns out to be “King shark”/”Mr. Big” and he takes the professor’s invention from the reporter. Rastapopoulos actually intends to keep his promise of setting children free, but, unaware of the bargain that has been made, they escape from their cell and hijack an underwater tank.
The villain takes over control of the tank with his computer, but he then notices Captain Haddock’s sub on his monitor. Rastapopoulos intends to make the children’s machine fire torpedoes at Haddock, which provokes a fight between Tintin and the other gangsters as he attempts to stop Rastapopoulos. At first, when the submarine commander crash lands on the controls, the computer goes haywire and abuses Rastapopoulos with its gizmos and gadgets. Then, Tintin accidentally kicks a gun on to the button that fires the torpedoes.
Rastapopoulos manages to regain control of the computer and tries to torpedo Haddock. Haddock evades the torpedoes but loses control of the sub. Niko manages to jam up the torpedoes with his shoe, preventing the tank from trying to finish off Haddock, but the tank is returned to the base by remote control. While waiting for the children to return, Rastapopoulos takes Tintin to his office and shows him his art collection, which consists of some of the stolen masterpieces. He further adds that with Calculus’ machine, they can make multiple copies of all the stolen masterpieces and sell them off for huge amounts of money.
Rastapopoulos tries out Calculus’ machine by cloning a cigar box, but the imitation starts growing almost trapping him and his henchman in the room. They barely manage to escape. Since the machine was faulty Rastapopoulos locks Tintin and the children in a chamber, intent on revenge. He then learns that police boats are patrolling the lake. He therefore decides to evacuate the base and orders his men to take all the art he has in the underwater city to the cave. He then floods the chamber with water. As soon as the water has reached a device high on the wall the base will auto-destruct.
Haddock manages to regain control of the disabled submarine and makes his way to the surface. On his way, he runs into two of Rastapopoulos’s divers and brings them up with him. Thompson and Thomson are in a boat looking for him when it is knocked over by the sub sending them into the water. Haddock, the detectives, and the divers are then picked up by a police boat commanded by the chief. Down below, with all of his men having evacuated the base, Rastapopoulos and the submarine commander are the last to leave the base in the submarine.
Back in the underwater town Tintin and the children manage to get free and escape through an airlock in life jackets, just before the base explodes. They reach the surface and rejoin their friends and the police. The police have captured all of Rastapopoulos’ men, but he himself has already crossed the border in his submarine. Since they are not Syldavian officials and therefore not bound by international conventions, Tintin and Haddock insist in going after Rastapopoulos in a motor boat. As they set off, Thompson and Thomson are caught in the motor boat’s cables and go for a rather panicky water skiing session before finally attracting Tintin and Haddock’s attention. Rather than get into the boat the Thomsons insist on swimming back to shore while the other two continue the search for Big Shark.
Rastapopoulos is near the shore. In order to pass the border posts and the rocks in Dragonrock Reef, he tries to navigate through an underwater tunnel, but forgets to lower the sub’s periscope which hits a low rock which causes the sub to crash against the rocks and get itself damaged beyond repair. The villains make for the surface as fast as possible. Tintin and Haddock sight the sub’s oil slick just as the crippled sub surfaces and runs aground on some rocks.
Rastapopoulos immediately blames the whole mess on the commander, but when the commander tries to point out it wasn’t his fault, Rastapopulos just fires him on the spot. They fall out just as Tintin and Haddock arrive. After a fist fight the heroes beat the villains and return to Syldavia. A Bordurian border guard (who’d been watching a football match between Borduria and Syldavia during the fight) at a nearby border post finds the broken sub and calls up a friend, whose crazy cousin likes to buy up scrap metal, with the intent on selling the sub for a huge amount of money.
Tintin, Snowy and Haddock return, to Calculus’ villa and are welcomed by a huge party of villagers who want to celebrate the end of the terror imposed by the gang (even Vladislav appears and congratulates Tintin as well as apologizing for taking him to be a prowler). Bianca Castafiore also turns up and tries to dance with Haddock. The Captain manages to escape the party and tries to have a quiet moment to drink some whiskey, but soon his peace gets disturbed again by Thompson and Thomson playing golf. They accidentally break his bottle but they all laugh over it.