After many sleepless nights, Muriel orders a new mattress. Muriel pays no mind to the grotesque-looking deliverymen (who hiss at Courage) that arrive on a sinister horse-drawn carriage, or the demonic face that appears on the mattress, and gladly accepts it. Courage attempts to warn his owners, but as usual, they are clueless about the oncoming danger. Muriel heads upstairs and lies down on the new mattress, locking Courage out of the room. Courage peeks through the bedroom window on a ladder and witnesses strange green vapors being emitted from the bed and possessing Muriel. She suddenly levitates above the bed, and as Courage watches in horror, he falls off the ladder.
Courage tries to explain his fear to Eustace, who of course fails to pay attention. Courage takes Eustace upstairs where they see Muriel transformed into a demon with fangs and wild, constantly flowing red hair. Concerned, Eustace and Courage go downstairs to figure out what to do. Courage goes upstairs to check on her, while Eustace believes the mattress just "gave her nightmares." Courage engages in a thumb war with the demon and loses, then flees from the room. Courage looks up on the computer how to get rid of the demon. It prints out an incantation for the demon. After Eustace's failed attempt, he gets vomited on by the demon, Courage then performs the exorcism from the computer and the demon unposseses Muriel. Then, Eustace comes in wearing a hazmat suit, and tries to perform the exorcism, but the demon possesses him instead. Muriel whacks him with the rolling pin, then she and Courage roll him up in the mattress and put a lock around it, and the mattress gets taken away by the evil deliverymen. The episode ends with Courage lying on Muriel's lap on the living room couch, as well as Courage saying his last line, "Now this is a bed I can sleep on any day."
- The titecard shows the Weasel having green fur and one large eye, but the creature is seen having purple fur and a glass eye in the episode.
- First and only appearance of the Mattress Demon.
- The wheels of evil deliverymen's wagon turning is a 3D animation.
- Incantation to exorcise a demon: "Hullabaloo, howdy doo! Musty prawns and Timbuktu! Yeltzy bye, hippity hoo! Kick 'em in the dishpan! Hoo hoo hoo!"
- In Queen of the Black Puddle, Eustace's newspaper says "Mattress Special", referring to this episode.
- This is one of the few episodes in which Eustace teams up with Courage to save Muriel without the exchange of money.
- This episode is a spoof of the 1973 film, The Exorcist, frequently called "the scariest film of all time," with Muriel as the character of Regan MacNeill and the Mattress Demon as Pazuzu. Several parodies of the movie's most famous scenes are done such as Muriel's sudden vomiting, when she rises from the bed, her head rotating 360 degrees, and the demon saying, "She's in here, with us."
- Certain scenes of this episode (such as the demonic head-spinning) may be censored in some countries.