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. 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, in old Newtonian runes, reading keep out. They break it open 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 with a female frog and a green toad.
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.
- 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
- Marcy keeps a pet venus flytrap on her nightstand.
- Anne realizes that she still has leaves and twigs in her hair for the first time.
- Sasha's influence apparently still affects Anne and Marcy.
- At one point, Marcy forgets which cartoon she is in when she says to Anne, "Let's get outta here, Scoob!"