Warning: Mild spoilers ahead. Nothing that will, in my opinion, ruin the story, but I thought it best to mention it, just in case.
'The Shadow Over Innsmouth' is a novella by H.P. Lovecraft, written in 1931, and part of the Cthulhu Mythos. The story concerns an unnamed protagonist visiting the town of Innsmouth, Massachusetts, which is populated by a group of inbred humans walking with a certain strange gait and having peculiar looks. Having to stay the night in the partially deserted town due to engine trouble with the bus he was supposed to travel with, he ends up talking to an old drunk, Zadok Allen, about the history of the town. He tells a fanciful story about fishmen living in the waters outside of Innsmouth, and how the town has for a long time cooperated and mated with these creatures. Though the narrator does not believe him, he is chilled by the implications of the story, and when someone tries to break into his room later that night, he decides it best to be on the safe side, and flee. What he sees while navigating the roads away from the town affects him drastically.
While the novella was written in 1931, it was originally rejected by the principal publisher of Lovecraft's works, the magazine Weird Tales. During Lovecraft's lifetime, it was only published once, as a slim book by Visionary Press. Weird Tales did print it, in an unauthorized abridged version, after Lovecraft's death.
Lovecraft's inspiration for the story is usually cited as Robert W. Chambers' The Harbor Master and Irvin S. Cobb's Fishhead. Both stories can be found mentioned in either Lovecraft's non-fiction work Supernatural Horror in Literature, or his letters.