Camp Rolling Hills is re-opened as Camp New Horizons one year after the events of Sleepaway Camp II, and Angela (Pamela Springsteen) sneaks in by posing as a camper that she bumps off in the opening scene. Bloody mayhem ensues when she realizes that this crop of campers is no better behaved than those in the first two movies.
Eventually, only Tony and Marcia are left alive. Angela decides to let the couple, who seem to be decent kids who have fallen in love, survive, and goes to climb into a car to drive off to parts unknown. Marcia gives chase and stabs Angela multiple times before Tony pulls her off and leads her away to find help. From the safety of a police car, Tony confesses his love for Marcia and offers to move to Ohio to be with her, only to discover that Marcia already has a boyfriend and isn't actually as nice as she has been pretending to be.
Meanwhile, in the ambulance, Angela comes to just as the two medics decide to let her die rather than save her life. She kills both of them with a hypodermic needle and the screen fades to black ...
Thanks Greg B!