Mystery novelist Cornelia Van Gorder rents a secluded mansion known as The Oaks for the summer. Shortly after settling in, she finds herself embroiled in a series of mysterious events. The small town is on edge due to a recent string of murders attributed to a ruthless criminal known as "The Bat," notorious for terrorizing individuals and leaving no trace. Cornelia's maid, Lizzie, becomes anxious amid rumors that the killer could be nearby. Meanwhile, the mansion's previous owner, bank president John Fleming, is entangled in a scandal involving embezzlement and long-hidden treasure. This intrigue grows when news breaks of Fleming's untimely death under suspicious circumstances.

As night descends, unusual happenings plague the mansion, deepening the mystery. Detective Andy Anderson arrives and starts piecing together clues, while Cornelia, an amateur sleuth in her own right, becomes determined to solve the case. She embarks on her own investigation with Lizzie in tow, exploring hidden passages and piecing together signs pointing to the possible involvement of wealthy suspects from the town. The tension mounts as Cornelia and her allies face increasing danger. They encounter strange figures and hear eerie sounds, suggesting they're being watched. With "The Bat" still at large and the treasure looming as a potential motive, the situation becomes increasingly perilous. As secrets unravel, Cornelia must use her wit and resourcefulness to uncover the truth behind the sinister occurrences at The Oaks.