On Halloween night in the small town of Strawberry Plains, a series of strangling incidents occur, just as children are trick-or-treating. More strangling incidents occur over the next several days, leading Sherlock Holmes-obsessed investigative journalist Wilhelmina Davies to visit multiple neighborhoods--despite bad storms and the huge risk to her personal safety--to try to get to the bottom of the incidents and find the strangler. She is assisted by a black cat, whom she spots in photographs at each of the crime scenes. The cat helps her by delivering notes to her from the strangler, and it eventually leads her to the strangler at her own home, though she survives a strangling attempt by stabbing the strangler in the stomach.
Wilhelmina meets the strangler, Michael Seabury, at the hospital at his request, though he refuses to provide her with a motive for his killings. However, after she leaves, he breaks out of the hospital and catches up with her on the sidewalk outside the hospital while holding a tube in his hands. But while she looks at a car crash that happened behind her, Michael vanishes.
After he vanishes, weird things happen in the town. While Wilhelmina follows the black cat to Old Parish Church, a bad storm gets worse, and she hears rustling in the trees, a shotgun being fired, a woman screaming, and owls hooting. The church would be even more haunting as it was long marred by a death there in 1501 of a pastor who died alone in the church under suspicious circumstances and strangely had symbols carved into his body with a knife. All over the walls inside the church were written pleas from desperate congregants, including "HELP ME," "SAVE ME FROM THE DEVIL," and "GOD, IF THIS IS MY LIFE, I WANT OUT." At times, Wilhelmina sees and hears children running around and playing outside of the church with one holding up a sign with a written message on it, the children being the ghosts of four children who died hundreds of years earlier.
Michael catches up with Wilhelmina at the church, still in possession of a tube. Wilhelmina stands up against him with Bible verses perfectly describing the man he was and what he did to the people he killed, angering him until he runs toward Wilhelmina and is shot dead midway by a police officer. The bad storm subsides, and the ghosts of the children run around the house. One child holds up a sign saying, GOD BLESS YOU, WILHELMINA, prompting her to smile.