All the youth of Mewni gather at the ancient Monster Temple to celebrate a new era of Monsters and Mewmans living together in peace, until guests start disappearing!
Star and Marco are refurbishing the temple for a party they are throwing for Mewman and Monster youths. While Star is nervous, she is happy to see them all get together so easily over similar interests. Nevertheless, she continues to fret over various things, becoming somewhat absent minded when Tom attempts to get intimate with her. Outside, Miss Heinous, Gemini and a fully reformed Rastacore wait patiently to strike. Inside, Tom tries to get Star to loosen up about current events. When Marco tries to explain that she is trying to focus her efforts on fixing Mewman/Monster relations, Tom becomes visibly disinterested and frustrated.
Meanwhile, Princess Spiderbite is approached by Slime Monster. While at first indifferent to him, she suddenly find him attractive when he reveals that his slime, which he compares to aloe vera, can immediately cure her giant spider bite. As they go deep into the temple together, they are attacked by something. Spiderbite returns, but everyone misinterprets the situation with the Mewmans accusing Slime of attacking her and the Monsters accusing the Mewmans of taking Slime and other monsters away. Star recruits Marco to go look for them, leaving Tom once again frustrated. The two go deep into the temple and find a hidden campsite that belongs to Mina Loveberry.
Mina reveals that she has been kidnapping the monsters as it is her sworn duty, but when Star tells her that she has been trying to repair the relationship between Monsters and Mewmans, she grows frustrated and prepares to attack. However, Miss Heinous and her minions arrive and prepare to drain Marco of his vitality, when her cheeks light up, revealing a secret nursery. Miss Heinous suddenly remembers that she is in fact Meteora Butterfly, daughter of Eclipsa, which Mina corroborates. Mina tries to attack her, but Star intervenes as she wants answers. Tom later arrives to help fight her off as Heinous is rescued by Gemini and Rastacore.
Mina attempts to look for Heinous, but Star stops her. The knights of Mewni and Rhombulus arrive, forcing Mina to retreat. As everyone is tended to outside, Rhombulus scolds Star for being careless, but refuses to discuss the significance of the temple. Tom admits that he did not realize how important the party was to Star, but will begin to respect her decisions. Star is further upset when the knights begin processing the Monsters, but not the Mewmans and demands they be let go. Meanwhile, Miss Heinous, who is starting to show monster traits, accepts that she is Meteora Butterfly.
- Eden Sher as Star Butterfly
- Adam McArthur as Marco Diaz
- Amy Sedaris as Mina Loveberry
- Jessica Walter as Meteora Butterfly
- Rider Strong as Tom Lucitor
- Jenny Slate as Pony Head
- Kevin Michael Richardson as Rhombulus
- Dana Davis as Kelly
- Gary Anthony Williams as Rich Pigeon
- Juliana Hansen as Princess Penelope Spiderbite
- Bobby Miller as Slime Monster
- This episode marks the third season's mid-season finale.
- This is the sixth half hour-long episode in the series, following "St. Olga's Reform School for Wayward Princesses", "Storm the Castle", "Bon Bon the Birthday Clown", "Face the Music", and "Starcrushed".
- Miss Heinous is revealed to be Meteora Butterfly, the Mewman/monster hybrid daughter of Queen Eclipsa and her monster husband.
- The Monster Temple is revealed to be the original home of Meteora and her parents.
- The episode's title is similar to, and a reference to, the 1962 novelty song "Monster Mash".
- One of the books at Mina's campsite is called Secret Tuxedo, a reference to Tuxedo Mask from the anime series Sailor Moon while another is called Thirty Shades of Magic; a reference to the novel Fifty Shades of Gray.