The story of The House on the Borderland is cast in the form of a manuscript written by an unnamed man, later referred to as the recluse, who lives in a large, ancient house with his sister and his dog. He writes the manuscript to document the strange events that he experiences, consisting of a mixture of visionary cosmic journeys and attacks by very real monsters.The first such experience begins when the narrator sees a green glow at one end of his study and soon develops into a bizarre journey in which he floats over a vast "plain of silence" and into a mountain-ringed amphitheater in which sits a house identical to his own except that apparently it is made of green jade. The surrounding mountains are inhabited by strange and horrible monsters, many of whom are identified as the models for some of the gods and demons of human history. He watches as a swinelike creature attacks the green house, as though seeking a way in. He then is drawn into space, with a view of the solar system, before being returned to his study.The next episode recounts a harrowing attack on the narrators home by swine-faced creatures that emerge from a cave in a nearby gorge. After several days of siege the narrator manages to drive them off. When the swine-things do not remount their attack, a peaceful period ensues during which the recluse explores the cave from which the monsters came. He finds that it leads to an enormous underground pit directly beneath the house and connected to it through a trapdoor in the cellar floor.The longest episode in the novel begins with the narrator standing at his window, the swine-things apparently forgotten. He hears a strange whirring sound that turns out to be the clock running at high speed, signaling an acceleration of time. As time speeds up, readers are presented with a scientifically precise description of the flickering of the seasons, then the rhythmic pulsing of the suns path from north to south, and eventually the expansion of the sun until it swallows up Earth itself. By this time the recluse has left Earth. He floats bodiless in space for millions of years, eventually coming upon a great green star out of which come an endless series of white globes. In one of the globes he finds his long lost love on the shore of the beautiful "sea of sleep." Again out in space, he perceives another procession of globes, this time red and full of blind, tortured faces. After passing again through the amphitheater of his first vision and seeing the green replica of his house, with damage matching that resulting from the attack of the swine-things, he is deposited back in his chair in his study. Although the house apparently has not aged, a pile of dust that was once his dog paradoxically attests the reality of his journey through time.A few days later, the house is attacked again by otherworldly creatures, though not necessarily the swine-things, that infect the narrator with a funguslike disease. The manuscript ends in mid-sentence, with the swine-things entering the house through the trapdoor in the cellar and climbing through the house to find the recluse in his study.