Exiting a lively gathering, Margot and friends jump in Lennon's car. Tragedy strikes when they hit someone walking on the road — it is revealed to be Alison, Lennon's twin sister. Amid everyone's panic, the group opts against involving the authorities, instead they transport Alison's body to a nearby cave.

One year later, Margot reunites with her friends. They start receiving threats from someone who knows what they did last summer.