In a remote part of Hawaii, a veterinarian named Doug Lambert enters an outdoor animal enclosure and is killed by a chimpanzee, an incident that ends with the animal tearing off his face and leaves the survivors reeling. 36 hours earlier, Lucy Pinborough returns to her island home after years away, traveling with longtime friends Kate and Nick, only to be surprised when Kate invites an additional guest, Hannah, without warning. During the flight, they meet Drew Charles Mann and Brad, two college friends eager to enjoy a Hawaiian party vibe.
Upon arriving, Lucy brings her friends to a cliffside house built into the rock, where her deaf father Adam communicates through sign language. Lucy’s sister Erin complains that Lucy has been away too long. The chimp, Ben Ben Pronsky, is more than a pet—he’s highly intelligent, trained to communicate with a custom soundboard on a tablet by Lucy’s late mother, a linguistics professor. Ben is delighted when Lucy gifts him a teddy bear, revealing the unexpected bond that ties him to the family.
That night, Ben’s behavior grows uneasy. Adam discovers a dead mongoose in Ben’s enclosure and learns the animal was bitten, prompting him to plan tests at a lab the next day and to ask Lambert to check on Ben before he heads off to a book signing. As the friends party by the cliff-edge infinity pool, the mood shifts when Ben becomes increasingly erratic and dangerous. Lambert is killed by Ben when the chimp escapes his enclosure, injecting a sudden dose of fear into the visitors’ carefree plans.
As everyone sleeps, Kate wakes to see Ben wandering aimlessly, and the teddy bear Lucy once cherished is stained with blood. The group discovers Ben acting aggressively toward Nick and Hannah by the pool. Lucy and Nick attempt to restrain the primate with a rope, but the plan backfires, triggering a vicious attack that leaves Erin wounded and the group plunging into the pool for safety. Ben, hydrophobic and unable to swim, roams the pool’s edge, watching and stalking his trapped guests.
The panic escalates when Ben leaps to the pool’s edge and Nick is killed by a forceful shove that sends him over the precipice. Ben continues to exploit every opportunity to inflict harm, and the group must improvise a path to safety using a large pool float as a makeshift barrier. When they awaken, Ben has vanished, and Kate ventures with Lucy to retrieve a phone from the living room, only for Ben to crush Kate’s head with a rock.
Meanwhile, Adam receives a lab update by text revealing the mongoose’s rabies positive result, but he chooses to ignore it, preferring to walk away from a potential film deal and head home after Lucy stops responding to calls. Drew [Charles Mann] and Brad arrive, drawn by the commotion but unaware of the danger inside. Drew is cornered by Ben in Lucy’s bedroom and is killed when the chimp tears his jaw apart. Brad locates the girls outside, but Ben assaults him and kills him with a shovel. Hannah sneaks out, grabs a phone and car keys, and runs to the driveway, only to mistakenly get into the wrong car. A frantic 911 call goes unanswered, and she is mauled to death when Ben uses the correct key fob to gain entry to the car.
Adam arrives to help Lucy and Erin, and the trio fights Ben, staving him off with a shattered wine bottle. They believe they have killed him and share a reunion-filled moment of relief, only for Ben to attack Lucy again, sending them tumbling over a balcony. Adam saves Lucy, and Ben falls to the ground, where a broken chair leg fatally impales him. Police and medical teams arrive after tracing Hannah’s call, and the family gathers in the yard to begin healing. Erin is taken to the hospital, while Lucy and Adam reassure each other that they will face the future together. As the dust settles, a chilling final moment arrives when an officer, examining Ben’s soundboard for evidence, accidentally plays the phrase “Lucy bad,” underscoring the lasting tension and fear left in Lucy’s wake.
