Barbra (Judith O'Dea) and Johnny Blair (Russell Streiner) drive to rural Pennsylvania for an annual visit to their father's grave. Barbara is attacked by a strange man (Bill Hinzman). Johnny tries to rescue his sister, but the man throws him against a gravestone; Johnny strikes his head on the stone and is left unconscious. After a mishap with the car, Barbara escapes on foot, with the stranger in pursuit, and later arrives at a farmhouse, where she discovers a woman's mangled corpse. Fleeing from the house, she is confronted by strange menacing figures like the man in the graveyard. Ben (Duane Jones) takes her into the house, driving the "monsters" away and sealing the doors and windows. Throughout the night, Barbra slowly descends into a stupor of shock and insanity.
Ben and Barbra are unaware that the farmhouse has a cellar, housing an angry married couple Harry (Karl Hardman) and Helen Cooper (Marilyn Eastman), along with their daughter Karen (Kyra Schon). They sought refuge after a group of the same monsters overturned their car. Tom (Keith Wayne) and Judy (Judith Ridley), a teenage couple, arrived after hearing an emergency broadcast about a series of brutal murders. Karen has fallen seriously ill after being bitten by one of the monsters. They ventured upstairs when Ben turns on a radio, while Barbra awakens from her stupor. Harry demands that everyone hide in the cellar, but Ben deems it a "deathtrap" and continues upstairs, to barricade the house with Tom's help.
Radio reports explain that a wave of mass murder is sweeping across the eastern United States. Ben finds a television, and they watch an emergency broadcaster (Charles Craig) report that the recently deceased have become reanimated and are consuming the flesh of the living. Experts, scientists, and the United States military fail to discover the cause, though one scientist suspects radioactive contamination from a space probe. It returned from Venus, and was deliberately exploded in the Earth's atmosphere when the radiation was detected.
Ben plans to obtain medical care for Karen when the reports listed local rescue centers offering refuge and safety. Ben and Tom refuel Ben's truck while Harry hurls molotov cocktails from an upper window at the "undead". Judy follows him, fearing Tom's safety. Tom accidentally spills gasoline on the truck setting it ablaze. Tom and Judy try to drive the truck away from the pump, but Judy is unable to free herself from its door, and the truck explodes, instantly killing Tom and Judy; the undead promptly eat the charred remains.
Ben returns to the house, but is locked out by Harry. Eventually forcing his way back in, Ben beats Harry, angered by his cowardice, while the undead feed on the remains of Tom and Judy. A news report reveals that, only a gunshot or heavy blow to the head can stop them, aside from setting the "reactivated bodies" on fire. It also reported that posses of armed men are patrolling the countryside to restore order.
The lights go out moments later, and the undead break through the barricades. Harry grabs Ben's rifle and threatens to shoot him, but Ben wrestles the gun away and fires. Harry stumbles into the cellar and collapses next to Karen, mortally wounded. She has also died from her illness. The undead try to pull Helen and Barbra through the windows, but Helen frees herself. She returns to the refuge of the cellar to see Karen is reanimated and eating Harry's corpse. Helen is frozen in shock, and Karen stabs her to death with a masonry trowel. Barbra, seeing Johnny among the undead, is carried away by the horde and devoured. As the undead overrun the house, Ben seals himself inside the cellar, where Harry and Helen are reanimating, and he is forced to shoot them.
Ben is awakened by the posse's gunfire outside the next morning. He ventures upstairs. A member of the posse mistakes him for one of the undead and shoots him through the forehead. The film ends with a photo montage of Ben as his body is thrown into the posse's bonfire.

Question: Why did Karen fall gravely ill?
Answer: Karen got bit by a monster.
Alternative answer: She was bitten.