It's summer and four friends Julie (Jennifer Love Hewitt), Ray (Freddie Prinze Jr.), Helen (Sarah Michelle Gellar), and Barry (Ryan Phillippe) are celebrating along with graduation. While returning home they hit a pedestrian walking down a curved corner and he dies. They decide to keep it a secret so that their future is safe. While dumping the body in a near by sea, the body becomes alive and tries to attack Helen. They throw him in and never return to that site. One year later, Julie she receives a letter stating that "I know what you did last summer." Julie is scared and gathers up her old friends who have all chosen their own ways and do not speak to each other anymore. They all decide to find out who saw them that night and in the process of tracking the suspect they see more and more people being killed.