The Dreams in the Witch House follows Walter Gilman, a student of both Mathematics and Folklore at the infamous Miskatonic University. Being interested in colonial history, especially the history of the witch trials of the late 17th Century, Gilman takes the opportunity to rent an attic room belonging to a legendary Arkham witch, Keziah Mason. Keziah Mason is well known for two things in particular: in 1692 she was imprisoned after being found guilty of being a witch, and managed to disappear without a trace; She also kept a familiar called Brown Jenkin, an oversized brown rat with disturbingly human features.
Gilman accesses the extensive occult library of the Miskatonic university to learn more about Keziah Mason. Other than the standard reading of the Necronomicon which we've come to associate with every Miskatonic student, he reads records of Mason's trial in which she confesses that, through the use of particular curves and angles one could pass into a space beyond our own.
This record intrigues Gilman, and he rents the eastern attic room in the old Witch House, which is known to have been used as an arcane laboratory by Mason. The room features walls and ceiling all meeting at odd angles, and peeling wallpaper revealing these arcane curves underneath.
Not long after renting the room, Gilman suffers a brain-fever and disturbing dreams, which he rationalises as being due to his intense study of mathematics and Mason's testimony of odd geometries which guide the way to alien spaces. In his dreams he plunges through a space of "inexplicably coloured twilight and bafflingly disordered sound". His motion through this space seems partially voluntary, though he finds that his arms, legs, and torso are missing or perhaps obscured by some odd trick of angles.
In his half-waking dreams before drifting into deep slumber Gilman often dreams of struggling to keep awake, his room being illuminated by a ghastly purple light, and that old familiar Brown Jenkin appearing from a rat hole. Gilman often nails tin over that rat hole the next day only to find that rats have apparently gnawed through the tin the following night.
Soon Gilman's dreams are haunted by the witch Keziah Mason, who speaks often on his need to meet the Black Man and go with them to the throne of Azathoth. Gilman recalls while awake that Azathoth is mentioned in the Necronomicon, and that the Black Man is one of the many preferred appearances of Nyarlathotep. In a following dream he witnesses "a rather large congeries of iridescent, prolately spheroidal bubbles" as well as a sentient polyhedron.
Gilman's downstairs neighbour notices the sounds of shod footsteps coming from the attic room late at night. Gilman begins to believe that he may be sleep walking, though his shoes are never disturbed. One night his neighbour enters the room to ask about a maths question, only to find Gilman gone. Gilman, having placed flour to track the presence of unwanted night visitors, finds evidence of no other visitors than his neighbour.
One night Gilman is transported to a cyclopean city of barrel-shaped creatures which we know from our previous reading as the Elder Things. In his dream he snaps a figurine of one of these creatures from a railing. The next day, the wife of his landlord comes across the same figure in his bed while cleaning. The following night he dreams of Brown Jenkin feasting on his blood, and awakes to a wound and dried blood on his wrist.
Gilman is now thoroughly convinced that he is being haunted. A concerned housemate offers to host him for a few nights. Gilman has a brief respite before the dreams start again. He dreams of accompanying Mason, Nyarlathotep, and Brown Jenkin in kidnapping a child for a dark ritual. A notable memory from his dream is walking barefoot through mud. He wakes the next morning in his old room, with caked mud on his feet. He is horrified to hear news of the kidnapping of a two year old the previous night, and descriptions of suspicious individuals matching himself, Keziah Mason, and Nyarlathotep.
Gilman and his housemate Elwood are deeply disturbed and do not sleep the following night. Finally we reach 30th April, Walpurgisnacht. In Arkham, Walpurgisnacht is observed and even feared by many residents. Gilman finds himself falling asleep through the day. That night, when he falls asleep once again, he dreams of hearing Mason chanting a dark rite.
Gilman follows the sound and finds himself suddenly in a room that must lie above his own attic room. Keziah Mason stands with sacrificial knife and bowl in front of an infant on a table. He launches himself at her, wrestles the knife from her hand, and leaves her unconscious on the floor. While this fight occurred Brown Jenkin has managed to spill the infant's blood, completing the ritual.
An alien chanting is suddenly heard, and Gilman knows that he must escape. He falls through the abyss and is guided back to reality by the prayers of his neighbour. He is found dishevelled and unconscious in his attic room.
The following night, he sleeps again on Elwood's couch. Elwood is awoken by the sound of scurrying rats and a disturbing sound coming from Gilman. Elwood is horrified to see a large bloodstain appearing on Gilman's blanket. Upon inspection, it appears that a rat-like creature had burrowed through Gilman's chest and eaten his heart.
The residents abandon the house and the landlord resolves to tear it down. One day in the attic room, workers knock down the ceiling to make a horrifying discovery. They find the bones of Keziah Mason and a child, as well as sacrifical dagger and offering bowl. Also among the debris they find the body of a giant rat with human features.
My favourite part of the story is that Gilman's odd room is immediately recognisable as a standard student room, at least to British students. No walls meeting at right angles, paint/wallpaper peeling, which was just hastily slapped over older wall furnishings by the landlord.
The congeries of iridescent spheres is often associated with Yog-Sothoth by Lovecraft readers. In fact, in most media adapting Lovecraft's work, a bubbling mass of spheres is frequently used to represent Yog-Sothoth. The sentient polyhedron is a creature that only appears in this story, and there is some really janky art for it.
Other than the spheres which I associate to Yog-Sothoth, we see a lot of returning creatures in this story: Azathoth, Nyarlathotep, Elder Things, and Shub-Niggurath.
Wikipedia only considers this related to the Dream Cycle and not an actual Dream Cycle tale, probably due to it not featuring pieces of the Dream Cycle continuity such as geography and books. I'm inclined to personally consider it a Dream Cycle story, as the primary plot is largely delivered via dreams.
Generally, this story was poorly received by Lovecraft's contemporaries, and so Lovecraft never submitted the story for publication. August Derleth had to submit it on Lovecraft's Behalf to Weird Tales, where it was published. Some scholars consider the story incredibly linear, at various points both too abstract and too explicit, and many rate it among the worst of Lovecraft's stories.