In the chilling tale of "The Thing on the Doorstep," Daniel Upton finds himself tangled in a web of horror and the occult. Upton, our narrator, begins by describing his best friend, Edward Derby, and their shared fascination with the dark and mysterious. However, as Upton delves deeper into Derby's life, he uncovers a series of unsettling events.

Derby's obsession with the occult leads him to marry Asenath Waite, a fellow enthusiast. Yet, as the years go by, Derby's behavior takes a disturbing turn. Upton becomes privy to Derby's strange stories, in which he hints at the possibility of Asenath's deceased father, Ephraim, being alive. Derby becomes increasingly agitated and fearful, fearing Asenath's power over him.

As the situation escalates, Derby is discovered in a state of incoherence in Chesuncook, Maine. During their journey back, Derby reveals his belief that Ephraim now inhabits Asenath's body. In a fit of instability, Derby abruptly changes personalities, leaving Upton unsettled.

Months later, Derby returns to Upton with a newfound solution to keep Asenath at bay. Despite this, Derby appears reluctant to leave his current residence, the Crowninshield House. His visits to Upton become more frantic, with Derby expressing his torment at Asenath's persistent presence in his mind. Concerned for his friend's well-being, Upton has Derby placed in a sanitarium in Arkham.

However, Upton's attempts to help his friend only uncover a chilling truth. Late one night, Upton receives a letter from Derby, who confesses to killing Asenath and burying her body in their cellar. In a horrifying revelation, Derby explains that Asenath's soul remains partly detached from her body and will continue to exist until the body is cremated. As Upton reads the letter, he realizes that the "thing on the doorstep" is none other than Derby himself, now inhabiting Asenath's decaying corpse.

Determined to put an end to the madness, Upton reluctantly follows Derby's instructions to kill him at the sanitarium. Fearful of his own soul being transferred, Upton must confront the terrifying consequences of his actions.

"The Thing on the Doorstep" is a haunting tale that explores the boundaries of life, death, and the supernatural. It serves as a reminder that the line between reality and the macabre can be easily blurred, leaving one to question their own sanity.