Last House on the Left

Teenagers Mari and Phyllis head to the city for a concert, then afterward go looking for drugs. Instead, they find a gang of escaped convicts who subject them to a night of torture and rape. The gang then kills the girls in the woods, not realizing they're near Mari's house. When they pose as salesmen and are taken in by Mari's mother and father, it doesn't take the parents long to figure out their identities and plot revenge.