Sarah (Kate Bosworth) isn't terminally ill, she just wanted to bring Abby (Katie Aselton) and Lou (Lake Bell) to Black Rock island to help her dig up an old time capsule they burred in their youth. On arriving they run into fellow former classmate (and younger brother of one of their familiar faces from childhood), Henry (Will Bouvier) an ex-soldier who has brought two of his former comrades in arms; Alex (Anslem Richardson) and Derek (Jay Paulson) on a so-called fishing trip. In fact they are there illegally hunting and were dishonorably discharged from events in the middle east that sound suspiciously to be almost war crimes. Nevertheless, Henry convinces Abby to take a nighttime forest stroll with him.
One thing leads to another, but their intimacy turns nasty when Abby rejects Henry, who in a jealous rage, starts raping her, during which she hits him with a nearby rock and escapes. Derek and Alex rush to his aid, but he dies in Alex's arms. Derek, infuriated that Henry is dead, persuades Alex to help hunt the trio down. The three women head for the shoreline to reach their boat, only to see their pursuers have set it adrift, and have taken up sniper positions, with one of them shooting Sarah in the head as they run along the sand.
Failing to swim for it, Abby and Lou strip off and huddle together for warmth while Derek tends to Alex's leg when he breaks it in a fall. Next morning, our duo retrieve their partially dry clothes then head down to the beach, looking to escape the island, when they come across Alex lying in a tent with a makeshift splint on his leg, as Derek patrols the shoreline, As they attempt to steal a gun, Alex comes to and raises the alarm, prompting Abby and Alex to struggle over the gun, which she snatches away after continuously striking his damaged leg. Despite him begging for mercy, she coldly blows him away.
Abby and Lou team up to fight with Derek, leading to hand to hand combat. Both women are stabbed and battered with rocks and sharpened sticks, before they finally overpower him by stabbing him in the neck, and then using his own combat blade to slice open his throat. Both women then leave on Henry's boat with a slain Sarah also on-board, having left the three men to the island's scavengers.