The story is presented as a first-person narrative using an unreliable narrator. He is a condemned man at the outset of the story. The narrator tells us that from an early age he has loved animals. He and his wife have many pets, including a large, beautiful black cat (as described by the narrator) named Pluto. This cat is especially fond of the narrator and vice versa. Their mutual friendship lasts for several years, until the narrator becomes an alcoholic. One night, after coming home completely intoxicated, he believes the cat to be avoiding him. When he tries to seize it, the panicked cat bites the narrator, and in a fit of rage, he seizes the animal, pulls a pen-knife from his pocket, and deliberately gouges out the cat's eye.
From that moment onward, the cat flees in terror at his master's approach. At first, the narrator is remorseful and regrets his cruelty. "But this feeling soon gave place to irritation. And then came, as if to my final and irrevocable overthrow, the spirit of perverseness." He takes the cat out in the garden one morning and ties a noose around its neck, hanging it from a tree where it dies. That very night, his house mysteriously catches fire, forcing the narrator, his wife and their servant to flee the premises.
The next day, the narrator returns to the ruins of his home to find, imprinted on the single wall that survived the fire, the apparition of a gigantic cat, with a rope around the animal's neck.
At first, this image deeply disturbs the narrator, but gradually he determines a logical explanation for it, that someone outside had cut the cat from the tree and thrown the dead creature into the bedroom to wake him during the fire. The narrator begins to miss Pluto, feeling guilty. Some time later, he finds a similar cat in a tavern. It is the same size and color as the original and is even missing an eye. The only difference is a large white patch on the animal's chest. The narrator takes it home, but soon begins to loathe, even fear the creature. After a time, the white patch of fur begins to take shape and, to the narrator, forms the shape of the gallows. This terrifies and angers him more, and he avoids the cat whenever possible. Then, one day when the narrator and his wife are visiting the cellar in their new home, the cat gets under its master's feet and nearly trips him down the stairs. Enraged, the man grabs an axe and tries to kill the cat but is stopped by his wife- whom, out of fury, he kills instead. To conceal her body he removes bricks from a protrusion in the wall, places her body there, and repairs the hole. A few days later, when the police show up at the house to investigate the wife's disappearance, they find nothing and the narrator goes free. The cat, which he intended to kill as well, has also gone missing. This grants him the freedom to sleep, even with the burden of murder.
On the last day of the investigation, the narrator accompanies the police into the cellar. They still find nothing significant. Then, completely confident in his own safety, the narrator comments on the sturdiness of the building and raps upon the wall he had built around his wife's body. A loud, inhuman wailing sound fills the room. The alarmed police tear down the wall and find the wife's corpse, and on its rotting head, to the utter horror of the narrator, is the screeching black cat. As he words it: "I had walled the monster up within the tomb!"

Question: What led the police to knocking down the wall?
Answer: A loud wailing in the room.  
Alternative answer: the cat wailing in it