Opening & Escalation: The movie opens with veterinarian Dr. Doug Lambert entering Ben's enclosure, where the rabid chimp brutally rips his face off. The story flashes back 36 hours. After Adam leaves for his book tour, Ben’s rabies symptoms worsen rapidly.
The Rampage: During the party, Ben breaks free. He kills Drew by tearing his jaw off and slaughters Brad. Recognizing Ben's rabies-induced hydrophobia, Lucy, Kate, Nick, and Hannah leap into the infinity pool.
Pool Siege & Deaths: Ben circles the pool, keeping them trapped. Desperate to get help, Hannah sneaks out of the pool to grab car keys, but Ben traps her in a vehicle and slaughters her. Ben also ambushes and kills Kate and Nick.
The Climax: Adam returns home. Because he is deaf, he does not hear his daughters' warning screams, allowing Ben to ambush him and bite his leg. Lucy and Erin leap out of the pool to save their father. Lucy grabs a sharp stake/rod and impales Ben through the chest as he lunges, killing the rabid primate.
The Ending: Emergency services arrive. As bodies are loaded into ambulances, police collect Ben's tablet soundboard, which glitched one last time to say "Lucy bad". Inside the ambulance, Lucy, Erin, and Adam share a subtle, traumatized, yet relieved smile—overjoyed to have survived together despite the destruction around them.