In Amity, Long Island, Ellen Brody (Lorraine Gary) still lives with her son, Sean (Mitchell Anderson), who is a police deputy. Ellen's husband, Martin -- the former chief of police -- died several years ago of a heart attack; Ellen is convinced his heart condition was caused by fear of the great white sharks he battled in the 1970s. One night just before Christmas, while on patrol in the harbor, Sean is ambushed by a great white shark disguised as a piece of driftwood. Ellen is devastated at her youngest son's death. Her eldest son, Michael (Lance Guest) arrives with his family -- wife Carla (Karen Young) and daughter Thea (Judith Barsi) -- arrive from the Bahamas, where he studies conch migration as a marine biologist. Ellen orders him to change careers as she no longer wants any of her family to have anything to do with the ocean. Instead Michael persuades her to come with them back to the Bahamas for Christmas. He promises that the ocean water is too warm for great white sharks, a species that Ellen is convinced has a vendetta against the Brodys.On the plane to the Bahamas, Ellen meets Hoagie (Michael Caine), a relaxed pilot with a gambling problem. They begin a flirtation. Michael and his colleague, Jake (Mario van Peebles) attach radio transmitters to conch shells. Carla makes ugly iron sculptures for the local government. Meanwhile, the shark that killed Sean has followed the family from Amity to the Bahamas. Ellen senses its presence, and the shark attacks the boat Michael uses as his headquarters. Michael elects not to tell anyone of his encounter with the shark but he and Jake decide to track the shark with their equipment. They put a radio tag on the shark but are not able to follow it through the ocean depths.Assuming the shark has left the area, Michael goes back down to study conch shells again. The shark appears out of nowhere and attacks Michael's submersible craft. Michael escapes into a sunken ship but the shark follows him. He barely escapes but insists on going down to look for the shark the next day. It's nowhere to be seen, as it has instead decided to attack the dedication ceremony for Carla's sculpture. It eats a woman who had taken Thea out on a banana boat. Thea makes it back to shore safely but Ellen steals Jake's boat and heads out to sea to confront the shark herself.Michael, Jake and Hoagie go after Ellen in Hoagie's plane. Hoagie tells them about Ellen's psychic visions of the shark's arrival. They buzz Ellen's boat just in time to prevent the shark from leaping out of the water and killing her. Hoagie crash lands the plane and, although at first it looks like the shark eats Hoagie, all make it to the boat safely. Jake and Michael realize they can use the radio tag and a big flashlight to shock the shark if it swims too close. Jake goes out on the gunwale to torment the shark with his magical invention but the shark leaps out of the water with a roar and snatches Jake into its jaws, breaking the gunwale.Ellen takes control of the ship's wheel. Michael uses Jake's magical invention to force the shark to roar like a tiger and leap from the water. Ellen rams the shark, impaling it on the broken gunwale, as she fantasizes about the shark attacking her loved ones and Martin's valiant stand against the first killer shark (even though she was not present to see it). The boat breaks in two and sinks. Jake resurfaces alive and with surprisingly minor bite wounds -- the shark had released him when it realized he wasn't a Brody.Ellen flies back to Amity, her fear of sharks overcome.
Output: Question: What does Ellen think caused Martin's heart condition?, Answer: His fear of the great white sharks he battled in the 1970s