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 there. Sylvester is already holding onto the bottom of Porky's coat, unwilling to let go, when he is spooked by a bat and jumps inside the coat. Porky chastises him for being afraid of the bat and says he is going upstairs to bed, Sylvester can go sleep in the kitchen. Unknown to Porky, Sylvester clings to him all the way to the bedroom and into bed. When Porky discovers him in the bed, he kicks him down the stairs, telling him to stay in the kitchen. Before long, Sylvester sees 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. Already shaking with terror, Sylvester's eyes go pin-point with fear and his heart visibly pounds in his chest. He races upstairs and into Porky's nightshirt. Porky begins scolding Sylvester, who interrupts this by demonstrating (in mime) what occurred downstairs. Porky criticizes the "ridiculous acting" and orders Sylvester back to the kitchen. Too frightened to comply, Sylvester pulls a gun from a dresser drawer and prepares to shoot himself in the head rather than face whatever fate the mice have in store for the pair. Porky disarms him and cannot believe how desperate his cat is.
Realizing he has no choice, Porky allows Sylvester to share the bed. Four mice push the bed out of the window, and it sticks on a pole. Porky, half-asleep and thinking it is cold in the room, asks Sylvester to close the window "like a good kitty". Sylvester proceeds to do so, himself barely awake and walking on thin air, as the pole springs the bed back into the room. Sylvester closes a tiny curtain on a birdhouse, gets back into the bed that isn't there and falls to the ground. He comes through the bedroom door with a big lump on his head. At that moment, he sees that the mice are about to drop an anvil on Porky from a crawlspace above the bed. Sylvester grabs the anvil at the last moment. Porky awakes and sees Sylvester poised above him with it in his hands. Porky questions Sylvester's intentions before dropping it on the cat's head and leading the way back down the stairs, heading for the kitchen. Sylvester sees the hooded mouse roll a bowling ball down the banister, targeted directly at Porky, who has reached the bottom. Sylvester races and shoves Porky out of the way - so hard he ends up in the kitchen headfirst in the cat basket - and is knocked unconscious by the ball.
Porky storms back from the kitchen (not noticing the basket being lowered below the floor) demanding to know why Sylvester pushed him like that. Seeing the cat knocked out, Porky suggests it is just a ploy to gain sympathy. Over the next few scenes, as he lifts Sylvester, carries him to the kitchen and puts him in the basket, a completely oblivious Porky barely escapes many attempts, via several tools and weapons, by the mice to kill him. Sylvester, out cold in the basket, is lowered below the floor just after 1 a.m. and is raised up again just before 4 a.m., without the basket and having turned completely light gray. Clearly traumatized, he makes his way to Porky's room where he wakes him up with pathetic mews. Porky is so startled he leaps into the ceiling light, then is angry, demanding Sylvester remove the "make-up", saying, "This is no time for comedy".
Porky, sick and tired of Sylvester's 'foolishness', decides to go into the kitchen by himself to show Sylvester there is nothing to fear. After a few seconds of silence, Sylvester takes a look in the kitchen, and witnesses the mice parading as they did the cat, only now it is Porky, bound and gagged and on his way to be decapitated. As the mice take him away, Porky holds up a sign 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 a star-tipped wand) appears. He magically produces an easel on which the word 'coward' is written; then, with diagrams and charts, he reminds Sylvester how Porky raised him from a kitten, shows him the "comparative sizes" of a cat to a mouse, and demands that he gets back in there and "FIGHT!". Suddenly bursting with courage after all this, Sylvester grabs a tree branch for use as a weapon, then decides to use the whole tree instead and races back into the mouse-infested house to fight. He sends the hundreds of murderous rodents running for their lives, much to his conscience's delight.
With the mice now all supposedly 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 behind Sylvester, wielding a mallet. Seeing this, Porky yells at 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 Lew Lehr caricature with a Napoleon army hat, and declares, "Pussycats is the cwaziest peoples!" and chuckles.
