Plot
A couple out sailing arrive at a small island, where the woman wanders off and stumbles across a fenced-off, seemingly abandoned compound and is attacked by unknown creatures.
Later, brothers Matt and John travel by seaplane to the island for a week of fun and relaxation. The uninhabited island was owned by their late uncle, who built a cabin on it, and they are joined by three friends: Nicki, Sara and Noah. The friends find a puppy, who mysteriously growls at Noah and runs outside the cabin. John and Sara pursue him and are ambushed by an older, vicious dog who bites Sara.
The following morning, the men are in the forest and encounter the man who came to the island at the beginning of the movie, who is now covered in blood. After he warns them about the dogs, a pack attacks and eats him. The group retreat to the cabin, and while Nicki is being attacked, John shoots an arrow at the dog but accidentally shoots through Nicki's calf. They are soon under siege by more dogs, one of which they impale with a fire iron.
The friends decide they must leave the island, but dogs have surrounded the seaplane. Lacking options and means of communication, they wait, but the dogs chew through the plane's mooring ropes, setting it adrift. John attempts to swim to the plane, which is covered in dogs who pursue him, but he reaches the dock. The group retreats to the house, where Sara begins to display canine behavior.
They decide to head to a storage shed and drive their uncle's Mercedes-Benz to the training compound to call for help. However, the car won't start, so they party through the night. After a power outage, Noah heads to the basement fuse box and is killed by dogs who have entered the house. Matt is bitten next, but he, John, Nicki and Sara hide in the attic, where they discover paperwork from the training compound and learn it was an Army facility to train attack dogs.
The next morning, the dogs have left. Matt and John manage to pop the clutch of their uncle's Mercedes, but Sara doesn't want to leave. The dogs attack her again and she falls out a window and impales both herself and the dog on a post.
Matt, John and Nicki drive to the abandoned facility and discover that the dogs were being genetically enhanced. John finds communications equipment, but a power cable on the antenna has been disconnected. Having been bitten and infected, Matt learns he can sense the dogs and realizes that if they escaped they can get back in. John fixes the antenna but is accidentally shocked by electricity, knocking him to the ground, where the dogs attack him. Matt shows up with a baseball bat to fight off the dogs, but a power surge causes a fire in the compound, creating a backdraft. Nicki lures in the dogs and the building explodes, killing all the dogs inside.
John tells Matt that he saw a nearby boat, owned by the couple from beginning of the film. The dogs surround the brothers but appear docile, and Nicki arrives in the Mercedes, picking up the brothers and speeding off. Swarmed by dogs, they can't stop the car, so John has Nicki drive the car off the pier, after which they swim to the boat and sail off. They decide to head to medical facilities before Matt and John can fall victim to the dog bites and become feral-minded like the dogs. In the last shot, the friends open the door to sleeping quarters and a stowaway dog leaps out.
