In the eerie and haunting tale of "The Rats in the Walls," we are introduced to the protagonist, Delapore, a man tormented by his family's dark history. After the tragic loss of his son in World War I, Delapore decides to relocate to the ancestral estate of Exham Priory in England. Little does he know that this decision would uncover a horrifying secret that has been lurking within the walls of the estate for centuries.

Upon moving in, Delapore and his trusty cat become disturbed by the incessant sounds of rats scurrying behind the walls. Curiosity gets the better of him, and he embarks on a journey of unraveling the mysteries that lie hidden beneath Exham Priory. With the help of Edward Norrys, his son's war comrade, and a team of academics, Delapore soon discovers that his family had maintained an underground city. This subterranean world served as a breeding ground for their insatiable appetite for human flesh. Generations of "human cattle," some reduced to a primitive state, provided sustenance to Delapore's ancestors.

However, this disturbing practice came to a ghastly end when Delapore's ancestor, Walter, massacred his entire family in their sleep, ultimately abandoning the estate to escape the perpetual horror. The remaining human livestock and one surviving relative fell prey to the rats that had infested the city's depths.

These revelations trigger a descent into madness for Delapore. Struggling with the weight of his family's dark history, his hereditary cruelty, and the grief over losing his beloved son, he succumbs to the darkness within him. In a chilling turn of events, Delapore attacks Norrys within the labyrinthine catacombs, consuming him while ranting in a mixture of archaic languages. His humanity deteriorates, reducing him to a frenzied state of animalistic grunts.

After a harrowing struggle, Delapore is subdued and confined to a mental institution. It appears that he is not the only victim of the horrors hidden beneath Exham Priory, as another investigator, Thornton, also succumbs to insanity. The decision is made to destroy Exham Priory, erasing any evidence of the underground city from existence. Yet, even in his confined state, Delapore continues to be tormented by the phantom squeaking of rats within the walls of his cell.

As the tale of "The Rats in the Walls" reaches its gripping conclusion, it is clear that the horrors of the past have left an indelible mark on Delapore's psyche. Whether it was the true nature of his family's lineage or the sinister presence of the rats themselves, the question remains: who or what is truly responsible for the unspeakable horrors that unfolded within Exham Priory?