"Lady Rainicorn of the Crystal Dimension" is the fourth episode in the eighth season of Adventure Time. It is the two hundred and twenty-ninth episode overall.
T.V. finds a hidden box and draws attention from a Rainicorn.
The episode begins with T.V. lying in bed blowing at a wind chime Lady Rainicorn then enters his room, asking him if he is going to get up as its noon, to in which he replies he's "good here" and then asks for money. Lady then tells him to get a job and leaves to visit Princess Bubblegum. While rummaging in the kitchen, he finds a crystal sandwich hidden in a box in a drawer and eats it. Then he takes the box to sell it. Lady returns with a doggie bag for him. She checks the computer and finds a message about what he went to do and flies out in a panic. At the marketplace, he tries to sell the box, but has no luck. Meanwhile, a rainicorn in a hoverbike looks for something. He releases a purple energy-feeler from a purple crystal, which searches the marketplace and finds him, and the (still undigested crystal sandwich inside him. The rainicorn, named Lee, asks about the box. T.V. wants money, but Lee offers a ride to a place where there is some and puts T.V. on his bike. Lady shows up and tells him he can't take 'it', but he turns around and throws the purple crystal in front of him and it opens up into a gateway. The gateway closes up behind them and collapses, but Lady simply tears a hole in the air with her horn and passes inside. They both arrive inside a crystal room. Lee is faster and gets through a crystal door that closes and she can't open.
In a city with many colored crystals, Lee and T.V. relax in a crystal mansion, where Lee is squatting. Lee secretly answers his phone and tells 'the boys' to come over. T.V. asks about a strange wheel, but Lee brushes him off. A group of dogs show up. Lady wanders around, coming upon a wall covered with anti-dog graffiti. She remembers a time when she dated Lee, who hates dogs. She later discovers his plans to steal something from a Dog military installation, the Labyrinth. She says it could touch off another war between rainicorns and dogs, but he convinces her to help him. They tie up and hide the two dog guards and take something called "The Mergence of Destruction." As he is talking she clubs him and takes it. He swears to get it back someday. She leaves the Crystal dimension by a sneaky way, emerging in the Candy Kingdom, seeing Cinnamon Bun being chased by two mushroom soldiers. She continues on, coming on a ruined barn, where she settles down and vows to keep it hidden for as long as possible.
In the present, she goes to a coffee shop and asks a rainicorn, Roy, where Lee is. She looks in the back, but finds only bags of coffee. She stirs up his latte and threatens his coffee bags to get him to talk. He finally tells her that Lee is hiding out in an old mansion, with a crew of revolutionary dogs. She leaves, taking his newspaper.
In the mansion, T.V. is being strapped to the hole in the wheel. One of the dogs turns a crank which spins the wheel and pops the Mergence out of T.V. and releases him. Just then, Lady shows up. Lee sics the dogs on her, but Lady hits them with the paper and they flee. He licks the Mergence and a yellow-orange energy tentacle spirals out, sucking him inside. The Mergence reaches out for Lady and T.V., but T.V. manages to put it in the box. Later, T.V. decided to stay in the Crystal Dimension for a while. Lady leaves on Lee's bike. T.V. briefly likes being independent, but is later seen at Lady's Parents house, settling in.
- Bob Rainicorn
- Ethel Rainicorn
- Cinnamon Bun (flashback)
- Toronto (flashback)
- Mushroom People
- Crystal Guardian
- This is the eighth episode (not counting the gender-swap episodes) in which both Finn and Jake neither appear nor are mentioned, the first being "Princess Day," the second being "Evergreen," the third being "Friends Forever," the fourth being "Be Sweet," the fifth being "Orgalorg," the sixth being "Varmints," and the seventh being "Broke His Crown." This is the first time Lady appears in an episode without Jake.
- This episode along with "I Am a Sword," "Preboot," "Reboot," "Two Swords," "Do No Harm," "Wheels," "High Strangeness," "Horse and Ball," and "Jelly Beans Have Power," are the only episodes to receive below one million US viewers on its premiere night.
- Lady Rainicorn and Lee's dialogue is subtitled throughout the episode, a necessity in this case, as the majority of the characters in the episode speak Korean.
- T.V. appears to have figures of the popular music duo Daft Punk over his bed.
- The motorbike that Lee rides has sideways wheels that allow it to float, similarly to Mario Kart 8 and Back to the Future.
- The brand of coffee seen in the cafe resembles Starbucks, which is very popular in South Korea.
- The Crystal Guardians from "Crystals Have Power" are shown walking down the street in the Crystal Dimension.
- This episode was previously titled, "Lady Rainicorn and the Crystal Dimension," and later "Lady Rainicorn in the Crystal Dimension," then "Lady Rainicorn of the Crystal Dimension." Both of them were working titles hence the word change.
- This episode reveals that some rainicorns can speak fluent English without the use of a universal translator.