A few days before the start of the summer season, the inhabitants of the small seaside resort of Amity are in turmoil at the discovery, on the coast, of the atrociously mutilated body of a young vacationer. For Martin Brody, the police chief, there is no doubt that the young girl was the victim of a shark. He then decides to ban access to the beaches but encounters hostility from the mayor, who is only interested in the influx of tourists.