The little ponies are setting up a celebration for the arrival of the Big Brother Ponies, who have been racing around the world for the past year and are due to come back tomorrow. However, before they can finish, a sorceress named Somnambula parks her wagon in the nearby woods and forces Kyrie, a caged canary, to sing. All the ponies who hear the singing are hypnotized, and head towards to her in a trance. The only ones left behind are Spike and the baby ponies, as the music seemingly only affects adult ponies, and Truly, who was directing the baby ponies' band and had the singing drowned out by the music.
Once they reach Somnambula and the singing stops, the ponies are very confused, as they thought they had heard a circus. Somnambula, who rapidly turns from an old crone to a young and beautiful woman, obliges by creating an illusory circus and leading the ponies to it. Once the ponies get there, each finds something tailored to her interests that draws her in and keeps her there. Meanwhile, Somnambula gloats about her growing power.
The Big Brother Ponies arrive the next day as scheduled, but are very surprised not to find anything or anyone. They meet with Spike, Truly and the baby ponies, who explain what happened, and decide to set out and find the missing ponies. The next night, however, the singing returns, ensnaring both Truly and the Big Brothers. Spike and the Baby Ponies hurry after them, and manage to snap the Big Brothers out of their trance by crashing a large snowball on their heads.
As the confused Big Brothers realize what's going on, the group sees Truly vanish through a golden door. The Big Brothers decide that that's where they need to go, but before they can pass it the door transforms into a snarling wolf's head...
This episode contains examples of:
- Antagonist Title: The episode is named after the witch who serves as its primary villain.
- Lotus-Eater Machine: Somnambula's illusory circus tailors its attractions to each pony sent there, presenting her with something that she can't resist partaking and losing track of time in, such as infinite ice cream for Lickety-Split and a giant garden for Posey.
- Musical Exposition: The episode opens with the ponies singing about the big brother ponies, explaining who they are and how they've been away for a year and are just about to come back home.
