Dennis goes on a dangerous journey to find Ludo living in another dimension.
The Avarius Family continues to receive all of Ludo's merchandise from when he tried to rule Mewni. Angered they toss it out, but Dennis, the youngest member, takes it to his room to discover a pair of dimensional scissors. He tries to run experiments to see where Ludo is, but it results in an explosion that alerts his parents. They go up to berate him and tell him that Ludo is not to be sought after. As they leave, Spider and Eagle are revealed to be living with Dennis. Spider has him simply open portals while she goes and looks for Ludo. She finds him and brings Dennis to a small planet that has a recreation of his house made out of garbage.
Ludo is shown to be happy and unbothered by the return of Dennis and Spider and invites the former in. It becomes apparent that Ludo has lost his sanity and has also recreated their parents from garbage as well. Dennis begs for him to come back home, but he seems to be convinced that they are home and that there is nothing wrong with their family life. Dennis manages to convince them to at least play charades (which Ludo pronounces as "sha-rods") before doing so. Ludo puts "his parents" to bed and Dennis breaks down how he is at his wits end having to put up with his parents' constant bickering and bad mouthing of Ludo. As he attempts to take his brother home, the garbage recreations of his parents materialize in front of them.
Dennis tries numerous times to get Ludo out of the house, but no matter how many times, the trash parents continue to appear. More copies of the parents appear (all of whom sound like Ludo impersonating them) and Dennis finally stands up for himself; destroying the trash copies and declaring Ludo his hero. Ludo thanks Dennis for finally standing up for him and they make up. Dennis invites him back home, but Ludo says that he has thing she needs to work out and that he can stand up to his parents now. As Dennis and Spider leave, Ludo opens a closet, revealing trash versions of Star and Marco and looks at them with sinister intent.
- Atticus Shaffer as Dennis
- Alan Tudyk as Ludo
- John DiMaggio as King Avarius
- Tress MacNeille as Queen Avarius
- It is revealed that, after the events of "Toffee", Ludo's eagle and spider went into hiding at Dennis's house.
- Some of the merchandise from "King Ludo" was sent to the Avarius family.
- The title is based on the 2000 film O Brother, Where Art Thou?