Porky Pig purchases a new home from a real estate agent, which turns out to be an old Gothic-style house: the sort featured in murder mysteries and ghost stories. His cat Sylvester is frightened of the creepy-looking place, but Porky finds it "quaint" and "peaceful", and looks forward to his first night in the place. Before long, Sylvester learns that the house is overrun with mice; killer mice, in fact (one wearing an executioner's hood and carrying an axe, the rest looking like the Chuck Jones-created characters Hubie and Bertie), who are just in the process of carting off the previous owners' cat (resembling a grey-furred version of Sylvester) to the chopping block. Sylvester's eyes get smaller, his heart beating, and races up to Porky's new bedroom.
Throughout the rest of the short, Sylvester is forced to dodge various knives, projectiles, trap doors, and other obstacles intended to kill him and his master. Porky, however, is completely unaware that anything is wrong, and is embarrassed that Sylvester is acting like such a coward. At one point, Porky is interrupted in his sleeping and scolds Sylvester who then explains what went on downstairs, but Porky orders him out, insisting that he sleeps in the kitchen. Because the mice have taken up primary residence inside the kitchen, where Sylvester does not dare to tread, the frightened Sylvester tries to shoot himself in the head with a gun, but Porky disarms him and tells him to cut it out. Having no choice, Porky allows Sylvester to sleep with him. Four mice push the bed out of the window, landing on a pole. Porky, thinking it is cold in the room, asks Sylvester to close the windows like a good kitty. Sylvester proceeds to do so after the bed launches right back into the room. Sylvester closes a tiny curtain on a birdhouse instead, and goes to sleep in thin air. He then falls and lands on the ground. He then comes back with a big lump on his head. Eventually, the mice are about to drop an anvil on Porky, but Sylvester stops it, resulting in an annoyed Porky who wakes up and sends him into the kitchen. Porky finds Sylvester unconscious (after Sylvester got hit by a bowling ball which was landing on Porky himself) and leaves him on a basket in the kitchen, but without notice, Sylvester is lowered down into the mice's lair while in the basket and a while later comes up to Porky who tells him to take off what Porky thought was make up, but the disguise was actually just Sylvester turning white from the aforementioned experience. Porky, sick and tired of Sylvester's 'foolishness', decides to show Sylvester what a coward he is by going into the kitchen himself. After a few seconds of silence, Sylvester peers into the kitchen. Sure enough, the mice have Porky bound, gagged, and on his way to be decapitated. The gagged Porky holds up a sign as the mice carry him away, which reads "YOU WERE RIGHT, SYLVESTER".
Out of fear, Sylvester scrambles out of the house. As he rests to catch his breath, his Conscience (a miniature Sylvester, wearing a wizard's robe and carrying star-tipped wand) appears and (in mime) deems him a coward. He magically produces an easel with diagrams and charts, reminding Sylvester of how Porky raised him as a kitten, showing him the "comparative sizes" of a cat to a mouse, and demanding that he gets back in there and "FIGHT!". Suddenly bursting with courage, Sylvester grabs a tree branch for use as a weapon before deciding to use the whole tree instead, races back into the mouse-infested house, and sends the murderous rodents running for their lives, much to his Conscience's delight.
With the mice now gone for good, Porky graciously apologizes to Sylvester and thanks him for saving his life. One leftover mouse (the executioner) pops out of the longcase clock with a mallet behind Sylvester. Seeing this, Porky warns Sylvester to look out, but the mouse clobbers Sylvester on the head, knocking him unconscious, much to Porky's shock. The mouse then yanks off his hood, revealing a Napoleon army hat, and declares (in a Lew Lehr voice), "Pussycats is the cwaziest peoples!" and chuckles.
Mel Blanc as Porky Pig, Sylvester, and Various MiceVHS - The Looney Tunes Video Show
LaserDisc - "Looney Tunes After Dark"
VHS - Looney Tunes Collectors Edition: Running Amuck
DVD - Looney Tunes Golden Collection: Volume 1 (The 1998 dubbed version, without dubbed notice and used on the DVD, restored the original titles.)