On a sun-soaked Hawaiian island, a routine day at a research-focused compound turns deadly when a calm, intelligent chimpanzee named Ben goes from curious to ferocious. A veterinarian named Doug Lambert steps into Ben’s enclosure, and as he reaches for a syringe, Ben vanishes. Lambert eventually lures him with a familiar teddy bear, but the moment Lambert turns away, Ben drags him back into the pen and brutally mutilates him. The shocking scene marks the beginning of a nightmarish chain of events that will test the family and friends who share Lucy’s home.
Flashback to thirty-six hours earlier, Lucy—a college student—flies home with her best friend Kate. The return is complicated by family tensions: Lucy has drifted away since her mother’s death, a rift that has widened with her younger sister Erin. Kate brings along Hannah, a mutual acquaintance, and two guys, Brad and Drew, who try to flirt with Hannah. At Lucy’s island residence, Erin lives with their deaf author father, Adam, and Lucy’s crush, Nick, joins the group along with Ben the family chimp, who is not only affectionate but surprisingly adept at using a communication pad. The gathering is lively, and Lucy makes an effort to reintroduce Ben to her friends while also attempting to bridge the gap with Erin.
That evening, the mood shifts at the pool. Hannah wanders into Ben’s reach, where the chimp’s now-lingering aggression surfaces. Ben drools and breathes heavily as he reaches for Hannah, initially restrained by Adam’s whistle and a quick, calm exchange. The next morning, Adam prepares to send a confiscated mongoose—believed to have bitten Ben—to a laboratory for testing, despite an uneasy undercurrent about the situation, including the academic signing of books with an interpreter present. Lucy notices tension between Nick and Hannah, hinting at the unsettled emotional dynamics within the household.
The tension escalates when Lambert returns to the home at Adam’s request and is killed by Ben, a brutal act that immediately destabilizes the group. With the chimp now unleashed, fear travels through the house. Kate wakes up to the danger; Ben finds her, and Lucy discovers blood on Ben’s teddy bear, heightening the sense that the danger is closer and more personal than anyone anticipated. Ben’s aggression grows, and his attempts to calm everyone down—like Lucy’s attempts to speak to him as she would a pet—fail. Erin is attacked with a deep bite to the leg, forcing everyone to flee into the water for safety.
The pool becomes a fragile haven as the group battles to stay afloat and find a way out. Nick attempts a risky plan to corral Ben toward the pool’s edge and push him off the cliff, but the chimp overpowers Nick instead, causing a fatal fall. With communication strained and fear mounting, Lucy grabs a phone but is confronted by Ben’s menacing presence when the chimp seizes control of the moment by using his pad to taunt her with a stark “Lucy bad.” Hannah provides a distraction, and Kate manages to free herself, yet tragedy continues to unfold as Ben vanquishes any sense of safety.
The nightmare crescendos as Hannah is violently attacked when Ben drags her by the hair, tearing away a portion of her scalp. The group splits up to seek help, first by summoning Brad and Drew, who misinterpret the situation as a casual hookup request. Ben attacks Drew in a bedroom, tearing through him in a brutal display of force, while Brad encounters Lucy, Erin, and Hannah and is later killed by the chimp with a weapon in his grasp. Hannah escapes briefly and grabs car keys, only to find herself trapped in the wrong vehicle as Ben closes in, fatally mauling her.
As the crisis deepens, Lucy and Erin attempt to reach a hospital, but Ben’s presence remains a constant, looming threat. Adam arrives, driven by worry for his daughters, only to discover the body of Drew. Ben stalks Adam, and Lucy and Erin lure the chimp back toward safety. In a tense balcony confrontation, Ben corners the sisters. Adam’s intervention becomes pivotal as he shouts and fights to protect them, culminating in a brutal struggle that leaves Adam mortally wounded. With courage in short supply and time running out, Adam sacrifices himself to save Lucy and Erin.
Responding authorities finally arrive, and medics move to extract Erin to the hospital. The distressing realization that Ben’s communication pad still calls out, “Lucy bad,” hangs in the air, though it’s revealed to be a grim echo as police collect the device for evidence. The family’s ordeal ends with the hope that Erin’s injuries are manageable and that Lucy, Erin, and the others will begin to process the trauma of what happened—and perhaps to understand the warning signs of a crisis that spiraled so rapidly from curiosity and companionship to a fight for survival.
