 It s the fourth of July and four friends Julie, Ray, Helen, and Barry are celebrating along with graduation. On their way home they hit a pedestrian walking down a curved corner. They make a pact to keep it a secret in hopes of saving their futures. While dumping the body in a near by sea, the body becomes alive and tries to attach Helen. They throw him in and never return to that site. One year later, Julie returns home from college for summer break. In the mail she receives a letter stating that "I know what you did last summer." Julie freaks out and gathers up her old friends who are now separated, miserable and not speaking. They all decide to find out who saw them that fateful night and in the process of tracking the suspect down, the body count goes up.