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In search of a calm environment conducive to writing, well-known mystery author Cornelia Van Gorder and her companion, Lizzie Allen, relocate to the country town of Zenith for the summer, renting the large vacant home of bank president John Fleming, known as “The Oaks.”  Upon settling in, however, Lizzie informs Cornelia that the newspapers are full of stories about a local murderer dubbed “The Bat” for his mysterious all-black attire and killing method of slashing the victim’s throat.

Shortly after arriving, Cornelia and Lizzie visit the Zenith bank, where vice president Victor Bailey expresses surprise that John would rent out The Oaks when he has always refused to do so.  Cornelia admits that she leased the house through John’s nephew, Mark, an insurance agent.

A little later, Victor and his wife Dale take aside a board member, police lieutenant Andy Anderson, to reveal Victor’s discovery that the bank is missing over a million dollars.  Anderson advises contacting John immediately, but Mark reveals his uncle is hunting with the town physician, Dr.  Malcolm Wells, and is unreachable.

At a secluded cabin, John surprises Malcolm by admitting that he has embezzled a million dollars from the bank and offers Malcolm half if he helps him stage his own death to throw off the authorities.  Malcolm asks John where they will get a body to double for John, and when the banker suggests murdering their guide, the doctor declines.  John then threatens to kill Malcolm to prevent him from telling anyone about his crime, but is distracted by the smell of smoke.  The men are startled to discover that the surrounding woods are on fire.  While John turns to gape at the blaze, Malcolm quickly grabs a shotgun and tells him that he prefers all of the money, then kills the banker.

Back at The Oaks a few days later, Cornelia reads of Victor’s arrest for the theft at the bank and also of John’s reported accidental death in a fire.  Lizzie reveals that since John’s death, there have been unusual noises throughout the house and all of the staff, except for the chauffer, Warner, have quit.  As a storm rises outside, the banging of shutters and creaking in the house make the women anxious.  Unknown to them, a man dressed in black lurks outside of the house, but when Lizzie spots the shadowy figure, Cornelia calls the police.  The man in black breaks into the house through the library door and hurries to the third floor.  The police arrive and, although unable to locate the intruder, confirm it is safe to go to bed.

Later, the figure in black sets loose a bat in the women’s bedroom and when Lizzie is bitten, Cornelia telephones Malcolm.  The doctor arrives and after examining Lizzie and the bat, assures them that the animal is not rabid.  Anderson comes to The Oaks shortly thereafter and, noting the break-in evidence, suggests that The Bat was the intruder and promises to assign a guard to watch the house.

The next day, Anderson visits Mark to relay his suspicions that despite Victor’s arrest, he believes that John was responsible for the bank theft and that he likely transferred the bonds into cash, which he has hidden in Zenith.  Mark admits having no knowledge of his uncle’s behavior, but reminds Anderson that bank secretary Judy Hollender is scheduled to testify with crucial information on Victor’s behalf at the upcoming trial.

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A few days later, Cornelia invites Dale and Judy to stay at The Oaks for the weekend.  The women are waited on by the new cook, Jane Patterson, and Warner, now acting as butler.  When the group begins discussing Victor’s upcoming trial, Cornelia announces her suspicion that John committed the theft.  Wondering if he hid the money in the house, Cornelia telephones Mark to ask if he has floor plans for The Oaks and he offers to come to the house later that evening to search for the plans.  That night as the women finish dining, Cornelia reveals her intent to write about The Bat and, unknown to them, Mark slips into the house.  He pushes aside a grandfather clock and opens a secret panel where the floor plans are hidden.  Before Mark can take the plans to Cornelia, however, he is attacked by The Bat, who has been watching from behind the curtains.  When the clock, which has never worked, begins striking the hour, Cornelia and the others come out and, spotting the panel door, open it to find Mark dead.  Malcolm, Anderson and the police are summoned and Anderson asks why Warner is absent.

Later that night, after the women have retired, The Bat enters through an unlocked door and cuts the phone line before going to the third floor where he begins hammering on a wall.  The muffled sound wakens the women and Cornelia discovers the phone line is dead.  Despite Judy’s entreaties, Dale insists on searching for the source of the sound and goes upstairs.  Finding The Bat in the darkened room, Dale rushes at him, but he escapes and kills Judy as he flees down the steps.  Attracted by the commotion, Anderson, who has been checking the grounds, rushes back inside and learns from Mrs.  Patterson that Warner is not in the house.  The butler appears moments later, claiming he was following Anderson until he was knocked out.  Malcolm abruptly returns, asserting his car skidded off the road.

Later, after Anderson leaves Detective  Davenport to guard the house, Cornelia visits the third floor to investigate where The Bat was searching and, discovering a secret room behind the fireplace, accidentally locks herself inside.  Meanwhile, The Bat visits Malcolm’s laboratory and is surprised by the doctor, who declares he knows his identity.  In the ensuing fight, The Bat shoots Malcolm.  Alarmed by Cornelia’s absence, Lizzie seeks help from Davenport and, spotting a light upstairs, they go to the third floor where Cornelia calls to them from the secret room.  Davenport opens the panel, freeing Cornelia, then notices a safe inside the little room.  The group then realizes the garage is on fire, but Cornelia declares the fire is a ruse to get them out of the house.  They wait and in moments, The Bat returns.  After wounding Davenport, The Bat turns on Cornelia and Lizzie, but is shot in the back by Warner.  Removing the killer’s black stocking mask, the group is stunned to discover The Bat is Anderson.