Plot
At a house in a remote part of Hawaii, a veterinarian named Dr. Lambert enters an outdoor animal enclosure and is attacked by a pet chimpanzee, which kills him by tearing off his face.
36 hours earlier, Lucy Pinborough, a young college student, arrives at an airport to return to her home in Hawaii after spending years away. She is accompanied by long-time friends Kate and Nick, but is dismayed to learn that Kate has secretly invited an additional guest, Hannah. While boarding, the friends meet Drew and Brad, two college-aged men looking to party in Hawaii.
The friends land in Hawaii and drive to Lucy's home, an elaborately designed house embedded into a cliff. Her father Adam, a famous novelist, is deaf and primarily communicates through sign language. Lucy's sister, Erin, complains that Lucy has been away for too long. It is revealed that the chimpanzee is Ben, the family's pet. Ben is highly intelligent and has been taught to communicate using custom soundboard software on a tablet by Lucy's late mother, a linguist. He is overjoyed when Lucy gifts him a teddy bear.
Later that night, Ben begins acting strangely. Adam discovers that Ben has been bitten by a mongoose, so he asks Lambert to check on Ben before leaving to participate in a book signing. As the friends party by the outdoor infinity pool installed at the cliff's edge, a now-rabid Ben kills Lambert and escapes his enclosure.
After everyone goes to sleep, Kate wakes up to find Ben shambling around mindlessly. Frightened, she alerts Lucy only to find Ben has vanished. Lucy then picks up Ben's teddy bear, finding it stained with blood. The girls hear a commotion coming from the pool and discover Ben acting aggressively towards Nick and Hannah. After getting Ben to settle down, Lucy and Nick attempt to restrain him with a rope. This backfires, triggering a violent outburst where Ben bites Erin, and everybody jumps into the pool. Hydrophobic and unable to swim, Ben watches the group and stalks the perimeter of the pool.
When Ben leaps to the cliff-facing edge of the pool, Nick attempts to push him off but is instead tossed over the edge by Ben and falls to his death. Ben finds ways to inflict various wounds on the others, until Lucy retrieves a large pool float that enables them to rest safely.
When the friends awaken, Ben has disappeared. Kate joins Lucy on an expedition to retrieve a smartphone from the living room, but is killed when Ben crushes her head with a rock. Adam worries when Lucy does not respond to his text messages, and leaves the event early when a lab notifies him that the mongoose's body tested positive for rabies.
Drew and Brad arrive at the house, having been invited by the girls earlier, oblivious to the danger. They search the house for the girls. Drew is cornered by Ben in Lucy's bedroom, and is killed when Ben tears his jaw off. Brad finds the girls outside, but Ben ambushes him and beats him to death with a shovel.
Hannah sneaks into the house and retrieves a smartphone and car keys. She runs outside but mistakenly enters the wrong car. Hannah frantically begs a 911 operator to send police to the house, but is mauled to death when Ben finds the keyfob for the car and enters the car's cabin.
Adam arrives and, with help from Lucy and Erin, fights off Ben, and Adam stabs him with a shattered wine bottle. As the family embrace thinking it is all over, Ben attacks Lucy sending her over the balcony but is ultimately saved by Adam and Erin, while Ben falls and is fatally impaled on a broken table leg. The police, having traced Hannah's call, arrive as the family recovers in the front yard. Lucy is startled when an officer, collecting Ben's soundboard for evidence, accidentally plays the phrase "Lucy bad".
