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At a house in a remote part of Hawaii, a veterinarian named Doug Lambert enters an outdoor animal enclosure and is attacked by a 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 best friend Kate, but is dismayed to learn that Kate has invited an additional guest, Hannah, without prior notice. While boarding, the friends meet Drew and Brad, two college-aged men looking to party in Hawaii.

The friends land in Hawaii where they meet up with Kate’s brother, Nick, 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 adopted pet; he is highly intelligent and has been taught to communicate using custom soundboard software on a tablet by Lucy’s mother, a linguistics professor, who died of cancer the previous year. He is overjoyed when Lucy gifts him a teddy bear.

Later that night, Ben begins acting strangely. Adam finds a dead mongoose in Ben’s enclosure and discovers that Ben was bitten by it. He plans to take the mongoose to a lab the following morning to run tests and asks Lambert to check on Ben before leaving to participate in a book signing. Later in the day, the friends party by the outdoor infinity pool installed at the cliff’s edge. As night falls, Lambert arrives and enters Ben’s enclosure, where a now-rabid Ben kills him and escapes.

After everyone goes to sleep later that evening, Kate wakes up to find Ben shambling around mindlessly. While Kate alerts Lucy, Ben vanishes. Lucy 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 calming Ben down, Lucy and Nick attempt to restrain him with a rope. This backfires, triggering a violent outburst where Ben bites through the rope, freeing himself, then proceeds to attack Erin and bite down on her leg. Assessing the danger, everybody jumps into the pool. Hydrophobic and unable to swim, Ben watches the group and stalks the pool’s perimeter.

When Ben leaps to the cliff-facing edge of the pool, Nick attempts to push him off, but is instead himself tossed over the edge by Ben, falling to his death on the rocks below. 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. Meanwhile at the book signing, Adam receives word from the lab indicating that the mongoose tested positive for rabies, which he dismisses since Hawaii is a rabies-free state. However, he abandons a potentially lucrative film deal and instead heads home when Lucy fails to respond to his text messages.

Having been invited by the girls earlier, Drew and Brad arrive at the house and search for the girls, oblivious to the danger. Ben corners Drew in Lucy’s bedroom and tears his jaw off, killing him. 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. She frantically begs a 911 operator to send police to the house, but is mauled to death after Ben finds the correct keyfob and uses it to enter the car.

Adam arrives at the house and fights off Ben with help from Lucy and Erin, stabbing Ben with a shattered wine bottle. Believing they have killed Ben, the family embrace until Ben attacks Lucy, sending them both over a balcony. Adam saves Lucy, while Ben falls and is fatally impaled by a broken chair leg. The police and doctors, having traced Hannah’s call, arrive as the family recovers in the front yard. Erin is taken to the hospital while Adam and Lucy reassure her that she will never be alone again. Lucy is startled when an officer, collecting Ben’s soundboard for evidence, accidentally plays the phrase “Lucy bad”