A couple at a neighbouring house are killed by an intruder in a lamb mask, who leaves the words "you're next" written in blood. Nearby, Erin accompanies her boyfriend Crispian to a family gathering at his parents' country house in Missouri. Crispian's parents, Aubrey and Paul, are joined by his siblings Drake, Felix and Aimee and their partners.
During dinner, a killer in a tiger mask shoots crossbow bolts through the window, killing Aimee's boyfriend, Tariq. Their phones are jammed. Aimee runs for help and is killed by a wire stretched across the doorway. A killer in a fox mask kills Aubrey. Crispian leaves to find help.
Erin fights back, killing Tiger with a meat tenderiser. She explains that she grew up in a survivalist compound. Paul discovers that the killers have been hiding in the house for days, and Fox slits his throat. Felix and his girlfriend Zee calmly watch: they hired the killers to murder the family for their inheritance. Felix then kills his brother Drake.
Erin sets traps around the house. She kills Lamb, then lures Fox into the basement and kills him. Felix and Zee attack her, but she kills Felix with a blender and Zee with a knife. When Felix's phone rings, Erin answers. Crispian, thinking he is talking to Felix, reveals he was part of the plan all along. When he returns, he tries to bribe Erin, and she kills him.
A police officer arrives, sees Erin over Crispian's body and shoots her in the shoulder. Ignoring her warning, he walks through the front door and is killed by the axe trap she set.
