[3][4][5][6] It was later adapted into a comic book with still images from the film used as illustrations[7] One night, in Brussels, Belgium, a pair of criminals discreetly break into the aquarium and steal a large priceless pearl.

But as it turns out, the crash was deliberately set up, as the pilot contacts his superior ("Mr. Big"/"King Shark") via a walkie-talkie, hidden in his binoculars, to inform him that the plan has failed.

Later, 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.

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 frogman who has obtained from Madame Black some plans stolen from Calculus' laboratory.

While Gustav leads Haddock to a pile of abandoned tires, 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.

The criminals leave behind a message on a tape player from their leader, "King Shark", who tells the heroes (with a voice which is startingly familiar to Tintin) that they will get the children back in return for Calculus' invention.

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.

Rastapopoulos promises to set Niko and Nushka free for the device, but, unaware of Tintin's arrival, the children escape from their cell and hijack an underwater tank.

While waiting for the children to return, Rastapopoulos takes Tintin to his office and shows him his art collection, gloating that with Calculus' machine, they can make multiple copies of all the stolen masterpieces and sell them off for huge amounts of money.

Haddock manages to regain control of the disabled submarine and makes his way to the surface, encountering Thompson and Thomson and the chief of the Syldavian police in a patrol boat.

In order to pass the border posts, Rastapopoulos tries to navigate the sub through an underwater tunnel, but forgets to lower the sub's periscope, which hits a low rock and breaks, causing the sub to crash and get flooded.

In the end, the Thompsons, Calculus, Niko, Nushka, and Gustav welcome Tintin, Snowy and Haddock back from their submarine vessel mission.