"The Thing on the Doorstep" is a short story written by H. P. Lovecraft, part of the Cthulhu Mythos genre of horror fiction. It was written in August 1933, and first published in the January 1937 issue of Weird Tales.
Two novels suggested as inspirations for "The Thing on the Doorstep" are Barry Pain's An Exchange of Souls (1911), about a scientist's invention that allows him to switch personalities with his wife, and H. B. Drake's The Remedy (1925; published in the U.S. as The Shadowy Thing), in which a character with the power of mind-transference comes back from the dead by possessing the body of an injured friend.
The story is divided into 7 chapters:
Daniel Upton, the story's narrator, begins by telling that he has killed his best friend, Edward Derby, and that he hopes his account will prove that he is not a murderer. He begins by describing Derby's life and career.
He then tells of Asenath Waite, and how Derby and she wed.
A few years later, people start to notice a change in Derby's behaviour. He confides in Upton, telling him strange stories of Asenath, and how he believes her father, Ephraim Waite, may not actually be dead.
Upton is called to pick up Derby who has been found in Maine, rambling incoherently. on the trip back, Derby tells of Asenath using his body, and suggests that it is in fact Ephraim who resides in the body of Asenath. Before finishing, he has a small seizure and rapidly changes personality, asking Upton to ignore what he might have just said.
A few months later, Derby shows up at Upton's door and says he's found a way to keep Asenath away; to stop her using his body. Derby finishes renovations on his old family house, yet seems strangely reluctant to leave Asenath's old place.
One time, whilst visiting Upton, Derby starts and begins rambling again. Upton gets him to sleep, but with his condition getting no better, ends up committing him to Arkham Sanitarium. Upton gets a call, saying that Derby has regained normality, though upon visiting, Upton can see it's not the true personality of Edward Derby.
Upton, at home, is greeted by a fetid-smelling dwarf-like figure at the door carrying a letter from Derby. The letter explains that Derby had in fact killed Asenath and buried her body in their cellar. Despite this, Asenath had managed to take control of his body while he was in the Sanitarium, meaning that "the thing on the doorstep" was actually Derby inhabiting Asenath's liquefying corpse. The note implores Upton to go to the sanitarium to kill Derby, who has been permanently possessed by Asenath-Ephraim's soul and that he should cremate his body to prevent this happening again. Upton does so, thus hopefully banishing Asenath's soul to the hereafter, though he reveals that he is afraid of having his soul transferred as well.