Tintin and the Lake of Sharks
Tintin and the Lake of Sharks is a 1972 French-Belgian animated adventure film based on The Adventures of Tintin, directed by Raymond Leblanc. It was not written by Hergé (who merely supervised), but by the Belgian comics creator Greg (Michel Regnier), a friend of Hergé. It was later adapted into a comic book with still images from the film used as illustrations.
One night, in Brussels, Belgium, a pair of criminals 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 criminals 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.