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 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. While boarding, the friends meet Drew and Brad, college-aged men looking to party. 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 passed away 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, before he leaves for a book signing, asks Lambert to check on Ben. 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 that evening, Kate wakes up to find Ben shambling around mindlessly. While she alerts Lucy, Ben vanishes. Lucy picks up Ben's teddy bear, finding it stained with blood. The girls hear a commotion from the pool and discover Ben acting aggressively towards Nick and Hannah. After calming Ben, 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 attacks Erin and bites her leg. Everyone 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 tossed over the edge by Ben, falling to his death on the rocks below. Ben finds ways to inflict 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 heads home when Lucy fails to respond to his calls and text messages. Having been invited by the girls earlier, Drew and Brad arrive at the house, 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 enters the wrong car. She 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 and fights off Ben with help from Lucy and Erin, stabbing him 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". ==Cast==