The Giant Squid attacks the Nautilus near the end of 20,000 Leagues Under the Sea. The scene with it has since become one of the most famous sequences in a Disney film.
The Attack
A badly damaged Nautilus was sinking in the ocean. After Captain Nemo's crew managed to stabilize the submarine, Nemo realized that they had gone "deeper than any man has gone before". The Giant Squid then moved towards the submarine. They were initially able to repel it, but the damage the submarine had taken made it harder to withstand the squid. Nemo decided the only thing to do was to travel to the surface, and fight the squid.
Nemo and his crew went onto the deck to fight the squid. Nemo was soon caught in one of the squid's tentacles, he was saved by the first mate, only to be caught again. Meanwhile, Ned Land managed to get onto the deck and managed to throw a harpoon at the squid, hitting it right between the eyes, severely injuring it. He then managed to cut Nemo free of the squid's tentacles.
Trivia
In a TV special made to promote the film, Walt Disney stated that "It's the squid who is the real villain." He also said that Peter Lorre, who plays Conseil, claimed "the squid got the part that's usually reserved for him".
The original version of the scene took place during sunset. It was ultimately rejected, and was eventually replaced with the version which appeared in the final movie. The 16mm footage of the original scene is still intact, and an edited version with added background music is available on the Special Edition DVD.
In the original novel, the Nautilus crew was supposed to lose one of their men due to the giant squid's attack before it was killed.