The Shadow over Innsmouth is a novella by H. P. Lovecraft in the horror fiction genre. The themes of the story are degeneration, tainted ancestry, forbidden mysteries that it would be best to remain unaware of, and a reality which human understanding finds both incomprehensible and intolerable.
The narrator is a student on a sightseeing tour of New England who learns of the existence of Innsmouth, a coastal town that following a strange epidemic and rioting in the last century fell into such decay and depopulation that it is not shown on maps. Although he is cautioned that Innsmouth is avoided because of "race prejudice" against the townspeople, and that visitors sometimes disappear or go insane, he decides to stop there for a few hours. He finds the inhabitants have a repulsive appearance and the whole town reeks of corruption, most houses are boarded up and apparently derelict.
Interested in folklore, the narrator locates an elderly local drunk known for his tall tales, and is told of how seafaring inhabitants of Innsmouth generations before voyaged to the South Seas where they found a cult that practiced human sacrifice to placate immortal undersea creatures. A pact with the creatures was made and they were brought to Innsmouth to dwell on an offshore reef and be worshiped; in exchange the undersea creatures provided gold. The townsfolk revolted against the human sacrifices, but they were defeated by a nocturnal invasion by the creatures. Those of the inhabitants not killed agreed to join the cult and keep its secrets. They were also forced to intermarry with the creatures; the children resulting from these marriages appear human, but become undersea creatures in later life.
The young man takes the old drunk's story as totally fantastic, but finds he must stay overnight, which he earlier had been warned against. He flees from the hotel during an attempt on his life, and finds himself pursued by strangely inhuman looking shapes, with reinforcements swarming ashore to aid in hunting him down. Leaving the town he finally sees them clearly in the moonlight, and confronted with the reality that they are a hybrid race, half-human and half an unknown creature that resembles a cross between a fish and a frog, the narrator blacks out. He escapes and calls in the authorities, who secretly destroy the town and torpedo the reef. However, the narrator has discovered that his own ancestry has its roots in Innsmouth, and thus he is fated to become one of the undersea creatures.
The story is divided into five chapters. In the first chapter, the narrator begins by recounting to the reader of a secret investigation that was undertaken by the government at the ruined town of Innsmouth, Massachusetts, and that the story told to them by the narrator himself is the reason for this investigation. He proceeds to describe in detail the events surrounding his initial interest in the town (antiquarian and architectural), which lies along the route of his tour across New England, taken when he was twenty-one. While he waits for the bus that will take him to Innsmouth, he busies himself in the neighboring town of Newburyport by gathering information from local townsfolk; all of it with superstitious overtones.
The second chapter details his ride into Innsmouth, described in great detail as a crumbling, mostly deserted town full of dilapidated structures and people who look just a bit odd and who tend to walk with a distinct shambling gait. All of this is offputting to the narrator, who describes the people as having the "Innsmouth look", "queer narrow heads with flat noses and bulgy, stary eyes". Only one person in town appears normal, a young clerk at the local First National grocery store who comes from neighbouring Arkham. The narrator gathers much information from the clerk, including a map of the town and the name of a local who might be a good source of information: an ancient man named Zadok Allen, known to open up about the town when plied with drink.
The majority of the third chapter is composed of the conversation between Zadok and the narrator. Zadok, who is very old, has seen much in the town and goes on at length, telling a tale of fish-frog men known as Deep Ones who live beneath the sea. It seems they bring prosperity in the form of fish as well as fantastically wrought gold jewelry to those who offer them human sacrifice. These fish-frog men are amphibious and are able to mate with humans. The hybrid brood have the appearance of normal humans in early life but, in adulthood, slowly transform into Deep Ones. The completed transformation brings them eternal life, which they live in cities under the sea. These fish-frog men were first discovered in the Indies by a native island tribe, which was itself found by an Innsmouth merchant named Obed Marsh. When hard times befell Innsmouth, Obed and some followers did what they could to call up the fish-frog men in their New England town. Zadok is at first angry that the narrator appears not to believe him. After seeing strange waves approach the dock, he becomes frightened and tells the narrator to leave town because they have been seen. The narrator leaves and Zadok disappears and is never seen again. When the story is over, the narrator is unnerved but thinks it a product of a fertile imagination.
Chapter four tells of the night that the narrator was forced to spend in town, after being told that the bus in which he came to town is experiencing engine trouble. The narrator has no choice but to spend the night in a musty hotel. While attempting to sleep, he hears noises at his door like someone trying to enter. Wasting no time, he attempts to escape out a window and through the streets, at times imitating the peculiar walk of the Innsmouth locals. Eventually he makes his way to some train tracks where he hears a great many creatures passing in the road before him. He hides and resolves to close his eyes, having at this point come to accept the idea that Zadok's story is true. He cannot keep them closed, however, and upon seeing the fish-frog creatures in full light for the first time, faints in his hiding spot.
In the final chapter, we hear of how the narrator wakes up unharmed and quickly walks to the next town (Rowley). Over the years that pass, he begins doing research into his family tree, discovering some disturbing information along the way. Eventually it becomes clear that he is a descendant of Obed Marsh himself and nightmares accompany the narrator's realization that he is changing into one of the creatures. As the story ends, the narrator, by then a student at Oberlin College, tells us that his horror at the idea is changing into acceptance, and that he will be quite happy living forever in the city Y'ha-nthlei, deep beneath the sea. He also has a cousin, even further transformed than he, being held in a mental hospital whom he plans to break free and take with him.