JEKYLL & HYDE meets THE HAUNTING OF HILL HOUSE in this dark and twisting thriller from the author of Bad Parts.

Kelly O'Neill hates many things-her dead-end job, her passionless marriage, her strained relationship with her daughter-but none more than the month of September.

Each year, a sinister letter arrives in her mailbox, a chilling reminder of her past misdeeds sent by a grieving mother. But this year, as the message reaches Kelly, a nightmarish entity attacks her daughter Emma within an eerie abandoned house. Though physically unharmed, Emma is left unable to walk after the incident.

As Emma struggles to recover, Kelly ventures into the attic and is attacked by the same mysterious force that targeted Emma. When Kelly comes to, she's face-to-face with her doppelgänger-an exact replica of herself, with the same appearance, voice, and mannerisms.

Determined to understand her daughter's worsening condition, Kelly captures the doppelgänger and demands answers. But instead of answers, she receives another vengeful letter in the mail. This time, the threat is clear-Kelly must confront her dark past or risk losing her daughter forever.

In a heart-pounding race against time, Kelly must navigate a treacherous alliance with her double and piece together the connection between the house, the letters, and their malevolent sender.