The Sleepover to End All Sleepovers

"The Sleepover to End All Sleepovers" is the first segment of the tenth episode of the second season of Amphibia, and the fifty-seventh episode overall.

It premiered on September 19, 2020, alongside "A Day at the Aquarium".

Synopsis
Anne and Marcy issue a Scare Dare Challenge that brings them, Sprig and Polly into forbidden areas of the castle.

Plot
Marcy has invited Anne, Sprig and Polly for a sleepover at the castle, much to the annoyance of Lady Olivia who still cannot tolerate the Plantars. Anne and Marcy realize that they never had a sleepover without Sasha as she is the "Queen of Sleepovers", but know that they have had enough experience to throw one for Sprig and Polly. They declare the sleepover mantra ("Pajamas, junk food and gossip galore/Pillow fights, movies and tearing up the dance floor/And listen up cus' this I won't repeat/never ever go to sleep). They all proceed to run amok throughout the castle; upsetting Lady Olivia who tells them to never go into the basement, but is interrupted by Anne, as the fun continues. As the gang relax, they realize that it is only 9:00 pm and Anne insists that they do a Scare Dare Challenge with the loser's name going into the Book of Losers forever.

Against Lady Olivia's warning, Anne suggests they head down into the basement. The kids sneak passed Lady Olivia, whom Polly draws on, and head down with them all refusing to turn back, lest their name end up in the Book of Losers. They enter hallway full of mirrors and find a blocked doorway with a sign saying "Shadow fish within, keep out", in ancient Newtonian runes. They break it open anyway and find themselves in a flooded room full of coffins. Anne and Marcy try to prove their bravery by going to one of the coffins and taking a selfie, upon which jellyfish-like ghost creatures emerge. They immediately deduce that they are deadly and flee the coffin room together. Along the way, Marcy notices a room full of growing plants that do not need sunlight and Sprig takes note of a destroyed painting of what looks like King Andrias.

The gang find a door that immediately enters Marcy's room, but the ghosts phase in anyway. They quickly discover that holding a mirror up to them makes them solid and push them back into the basement. The kids refuse to go to sleep and stay awake until morning. After wondering so many questions, they agree that the sleepover was great and all four of them add their names to the book. Anne and Marcy reveal that they have added their names numerous times and that Sasha always won. Anne states that she needs to make up with her. Lady Olivia angrily enters to tell them that their meeting with King Andrias is soon. They laugh and a ghost returns to laugh with them, causing them all to scream.

Cast

 * Brenda Song as Anne Boonchuy
 * Justin Felbinger as Sprig Plantar
 * Bill Farmer as Hop Pop Plantar
 * Amanda Leighton as Polly Plantar
 * Haley Tju as Marcy Wu
 * Michelle Dockery as Lady Olivia

Mentions

 * Sasha Waybright
 * King Andrias

Trivia

 * Anne realizes that she still has leaves in her hair for the first time.
 * A Moss Man having a chain on its neck is shown in the basement's room full of plants very briefly.
 * This reveals that Marcy and Anne usually took orders from Sasha to do anything fun.
 * The 'ghosts' from the castle's basement might be the same species as the basement ghosts Hop Pop mentioned in the episode "The Domino Effect".
 * However when fully translated, the warning sign on the basement door reads "Shadowfish Within- Keep Out", indicating that the mysterious creatures with floating, phasing and extreme acidic abilities are collectively known as Shadowfish, not ghosts. And the Shadowfish can be defeated by making them look at mirrors to make them from phantom form to solid.
 * A few things are revealed in the castle basement:
 * A hall of mirrors.
 * A room full of plants.
 * A torn picture of King Andrias with an unknown toad and an unknown frog with the same skin color and hair as Sprig.
 * Marcy keeps a pet Venus flytrap on her nightstand.
 * Sasha's influence apparently still affects Anne and Marcy.
 * Anne, Sprig and Marcy spend the entire episode in their pajamas.

Allusions

 * Donald Duck: The back of Marcy's phone has an image of a duck wearing a blue hat, similar to Donald.
 * : Marcy after confronting the Shadowfish exclaims to Anne, "Let's get outta here, Scoob!"
 * : The shot where a moth landed on Marcy's mouth on the mirror reflection is a reference to the movie's poster.