Porky Pig purchases a new home from a real estate agent, which turns out to be a dilapidated old house. His cat Sylvester is horribly frightened of the creepy-looking place, but the clueless 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.
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 was interrupted in his sleeping and said to Sylvester "Sylvester! I thought I told you to..." before his cat explains what went on downstairs but Porky said "What dishon...dishon... what ridiculous acting. Get out! (Sylvester points as if to say "Me?") Yes, OUT!!" Sylvester tries to shoot himself in the head, which alarms Porky, who tells Sylvester to "stop that, gimme that gu-gu-gu-pi-pistol!" - finally taking the gun after a short fight.
The mice have taken up primary residence inside the kitchen, where Sylvester does not dare to tread. Porky finds Sylvester fainted (after Sylvester got hit by a bowling ball which was landing on Porky himself) and leaves him on the basket, but without notice, Sylvester has been lowered down into the mice's lair while in the basket and a while later comes up and Porky 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, declares that "I'm going into that kitchen myself, and prove what a yellow dog of a cowardly cat you really are!" Sylvester feels humiliated and Porky goes into the kitchen. 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 (Porky holds up a sign as the mice carry him away, which reads "YOU WERE RIGHT, SYLVESTER".)
Sylvester, in fear, scrambles out of the house, not stopping until he is a good half-mile away. As he rests to catch his breath, his conscience appears and (via signs, not dialogue) cuts him to the quick, calling him a coward, reminding him of how Porky raised him from a kitten, showing him the "comparative sizes" of a cat to a mouse, and demanding that he get back in there and "FIGHT!" Suddenly bursting with courage, Sylvester (grabbing a tree branch for use as a weapon, and then changing his mind and taking the whole tree), who's changed from the scaredy cat to a real hero, races back into the mice-infested house, fights at full power and sends the hordes of murderous rodents running for their lives.
Having got rid of those mice, Porky graciously thanks Sylvester for saving his life, just as one leftover mouse (the executioner) pops out of the longcase clock and clobbers the cat with a mallet. The mouse yanks off his hood, revealing a Napoleon army hat, and declares (in a Lew Lehr voice and imitation), "Pussycats is the cwaziest peoples!" and chuckles as the film rises out.
At one point Porky says he should have gotten a dog. This is in stark contrast to several other Porky cartoons directed by Chuck Jones, where Charlie Dog tries to get himself adopted by Porky, who says he doesn't want a dog. As this short was released after Little Orphan Airedale, this could be Porky saying he regrets not adopting Charlie.
This is the first cartoon where Sylvester is given his name, as prior to that he was unnamed (or in the case of Tweetie Pie he was named "Thomas").
One version has the Blue Ribbon Merrie Melodies title card and cuts the scenes where Sylvester produces a pistol from a dresser drawer and threatens to kill himself if Porky does not let him stay to protect him from the mice and the scene where Porky bends over to pick up a scared Sylvester and nearly misses being shot by a mouse in a black hood. This was also how the scene was cut when aired on ABC (as part of The Bugs Bunny and Tweety Show) and the former WB! network.
A rarely shown 1998 dubbed version, with the original opening and credits, which first aired on a 4:00am showing of the Cartoon Network compilation show Bugs and Daffy in 2003 and aired again on a New Year's Day marathon of Looney Tunes cartoons on January 1, 2010, has the original title cards and credits and reinstates the scene where Porky bends over to pick up a scared Sylvester and nearly misses getting shot by a mouse in a black hood, but still edits the scene where Sylvester withdraws a pistol from a drawer and threatens to kill himself with it and fights with Porky over the gun. In contrast to the usual "dissolve-edit" version that aired frequently on Cartoon Network, the new edited version cuts from Porky asking Sylvester to leave his bedroom to Sylvester crying and Porky chastising him for being a crybaby before relenting, and crops the shot of Porky chastising Sylvester for crying so the gun behind Porky's back is never shown. As of 2011, this new edited version is the version that used to air on Cartoon Network and now airs on Boomerang.