As Amity Island's Police Chief Brodie, it is your responsibility to stop the lethal shark Jaws from killing the residents and tourists. The Mayor is keeping an eye on you, and wants the job done as quickly and cheaply as possible.
Unfortunately, the main weapon which must be used to finally kill Jaws has been broken into four pieces, which are located along the sea bed in different places. As you explore the 150-plus screens of the game, you must shoot the other fish - some can be shot using your standard weapon, while others (those which flash) must be shot using special limited bullets.
You also have responsibility for closing beaches, which removes the risk of a shark attack, but also costs the area business, especially on hot days, and irritates the Mayor.