Finn, Jake, and some friends attend a dinner with a mysterious host. However, the host then reveals himself as a murderous ghost that will possess the body of one guest to kill everyone else. The night turns from a masquerade party into a terrifying whodunit as the guests start to get murdered one by one.
Finn and Jake head to a castle for a party they were invited to by an unknown host. Outside the door is a box of disguises for them to wear. When they head inside, they find Princess Bubblegum, Lumpy Space Princess, Cinnamon Bun, and BMO inside, under the guises of "Lady Quietbottom" (Princess Bubblegum), "Duchess Gummybuns" (Lumpy Space Princess), "Guy Farting" (Cinnamon Bun), and "Professor Pants" (BMO), while Finn and Jake receive the names "Prince Hotbod" and "Randy Butternubs." They discover a note on Cinnamon Bun's back. Finn reads it, telling them that their host is a ghost that will take over one of their bodies and kill the rest of them. Cinnamon Bun tries to leave, but as he tries to open the door, the lights flicker. When they come back on, there is nothing left of him but a skeleton.
Jake thinks Finn planned this from his birthday prank but Finn denies it. Finn brings the remaining party guests to the staircase and has BMO take pictures of them with its ghost-detecting gear to see who is possessed. Jake does not blink when his pictured is taken, making everyone believe he is the ghost until BMO confesses it has no ghost detection gear and just likes taking pictures.
Jake suggests that they get candles in case the lights go out again, which they do. He and BMO head into the basement where BMO disappears, having been captured by what appears to be a long worm-like white ghost. Jake then runs upstairs and tells Finn and Princess Bubblegum that the ghost has captured BMO but Finn accuses him of being the ghost. Everybody panicked when they heard Lumpy Space Princess screaming from the bathroom (which turns out she was using it).
They go to the dining room where they begin to point fingers at each other. Finn asked Jake a question that only the real Jake knows. After Jake correctly answers the question, Jake accuses Lumpy Space Princess and that she has to tell them why she broke up with Brad to prove herself. She refuses on the grounds that they did not know what happened between them. Finn wanted to tie her up with Princess Bubblegum agreeing with him, causing her to be accused of being possessed (Lumpy Space Princess then continually interrupts; confessing that she broke up with Brad because he kissed her on the mouth and she was not ready).
Finn then asks Princess Bubblegum to recite what he wrote her two years ago as a birthday card, but she refuses because she could not remember. Then the long worm-like thing-the 'ghost'-passes through the roof and through Princess Bubblegum, causing her to melt. Finn checks the closet where Cinnamon Bun and BMO are supposed to be hiding, but instead finds their splattered and scattered remains draped all over the walls and ceiling of the closet. Finn then admits that this was a prank, but that was not part of it. The 'ghost' also engulfs Lumpy Space Princess and she tries to call her parents but then the lights start to flicker and the ghost captures her and puts her in a painting. Finn and Jake tries to escape but the doors were locked. Then they try running upstairs but Jake was captured. Finn then hides in a room. Out of bookshelves, the Ghost Lady appears and tries to get Finn but he managed to escape by breaking a hole in the wall. In the next room, the floor bursts open, a picture of crows on the wall comes alive and releases crows into the room. The floor separates completely into a mouth shape, and the furniture begins to try and eat him, and Finn flees this room and up a nearby stairwell into the observatory.
The observatory is surrounded by ocean, which crashes up against the base of the building in huge waves. The light rises through the floor, finally cornering Finn, when the light takes the sheet off its head and herself to be Lady Rainicorn with all the guests fused in her body, and Jake on her back. Jake reveals that he knew about Finn's plan for months and 'flipped the script' with Lady Rainicorn. Finn asks about how PB melted (it was a wax dummy), and then the monster he saw and the house coming alive. Jake believes he was just imagining it out of fear but Finn denies it. Jake then thinks its just another joke. After they leave on LR, Finn flashes back to the monster he saw and decides to put it in "the vault" and forgets about it.
- Finn/Prince Hotbod
- Jake/Randy Butternubs
- Princess Bubblegum/Lady Quietbottom
- Lumpy Space Princess/Duchess Gummybuns
- Cinnamon Bun/Guy Farting
- BMO/Professor Pants
- Lady Rainicorn (as the ghost)
- A ruined skyline can be seen outside the mansion, which is are remnants of the Mushroom War.
- Cinnamon Bun may have feelings for Lumpy Space Princess, as demonstrated when he says, "Now's my chance," when he hears that she has broken up with her boyfriend.
- At the top of a staircase, a portrait's plate reads: 8•18•12. This (August 18, 2012) is the date background painter Santino Lascano was married.
- Finn's mask is a fox mask, Jake's mask is a classic green eye-mask, Princess Bubblegum's mask is a swan mask, Lumpy Space Princess's mask is a red triangular mask, BMO's mask is a bear mask, and Cinnamon Bun is not wearing a mask; although he is wearing a mustache, a top hat, and a monocle.
- On the bottom of a painting, it says "La Cazza [sic] Del Zorro" which is Spanish for "The hunt of the fox."
- This is the first episode where Finn wears long pants other than his pajamas.
- In the Adventure Time With Finn and Jake: Righteous Rules For Being Awesome book, it mentions it was Jake's birthday.
- On the painting where a lady is hugging a bull, the portrait plate reads MAYO.11 which is Spanish for May 11.
- When the episode was first aired on October 17, 2011, when BMO and Jake go down into the basement, BMO finds a lighter. When Jake lifts the lighter above his head and out of view, this is when he lights it. However, when this episode was shown again on Cartoon Network, the lighter is changed to a yellow glowstick for unknown reasons. Later in the episode, Jake still says he dropped the lighter in the basement and refuses to go down there again, even though it was changed to a glowstick.
- When Jake states that Finn's dream includes a half-orc shaman, this is a reference to Warcraft's Thrall, who is also a half-orc shaman.
- Jake's trick in "Mystery Train" is alluded to at the beginning of the episode. Also, at the end of the episode, Jake refers to the same events by saying "I gotcha, dude! Two for two!"
- Brad from "Trouble in Lumpy Space" is mentioned. Lumpy Space Princess reveals why she broke up with Brad in the first place.
- The mysterious ghost lady who appears in the climax will appear multiple times after this, including as one of Finn's fears in "King Worm"; the truth of who she is will be revealed in "The Vault," which is Shoko.
- The mansion resembles the one in which Big Destiny and the Destiny Gang live in "Finn the Human," as well as the one in which the Bath Boy Gang live in "The Vault." This would make sense, since Shoko died on a mission for them, and later appeared there (though it could that be she was just following Finn to get him to return a thing to someone).
- The theme of using a haunted house in a game of subterfuge and deception is likely taken from the 1959 film House on Haunted Hill. In the film, Annabelle Loren and her lover David Trent conspire murder Annabelle's husband, Frederick Loren, by faking a haunting. Frederick becomes aware of this beforehand though, and arranges his own ghost-inspired deception which culminates in the death of both Annabelle and David. This mirrors how Jake becomes aware of Finn's ruse, and turns it around on him.
- Finn mentions about "elevator dreams" when he was asking Jake, a story which was created by Roald Dahl.
- This episode is done in a Whodunit mystery style, where strange murders occur within a confined space, like the murder mystery book And Then There Were None, by Agatha Christie, or The Mystery of the Yellow Room, by Gaston Leroux, who wrote The Phantom of the Opera.
- The note on the box of Finn and Jake's masks said Wear Me, which is similar to the Alice in Wonderland phrase that was written on the cake and drink: Eat Me & Drink Me.
- The title card may also be a reference to the game Clue, since the various murder weapons the guests are holding and their outfits resemble those of in the game. Princess Bubblegum has the knife, Lumpy Space Princess has the rope, Jake has the gun, Finn has the fireplace poker, and BMO has the candlestick.
- The episode's set-up also shares a number of elements with the 1985 film, Clue. In both, the guests receive anonymous invitations to a party held by an absent host, and they are assigned pseudonyms. There is also a part where the guests split up into pairs to search each floor of the house.
- The opening scene, costumes, and much of the interior of the mansion are drawn in the pseudo-Victorian style of the late Edward Gorey, a prolific author/cartoonist best known to the general public as the creator of the animated sequence opening the PBS Mystery series and his darkly humorous books.
- The ghost appearing near the end is drawn and painted in the style of traditional Japanese 'ghost paintings'.
- The mask BMO wears resembles the bear mask from the film The Shining. The Shining is also a ghost story set in a largely empty hotel.
- When LSP is sent into the painting, it is a reference to the Roald Dahl book The Witches where the girl is sent into the painting.
- The letter's mention that their Host is a Ghost is a reference to the Ghost Host from the Disneyland ride, The Haunted Mansion. In addition, "I am your Host. I am a ghost." sounds similar to a similar line spoken by the Ghost Host of the attraction when addressing to the audience: "I am your host, your Ghost Host."
- When Lumpy Space Princess enters in the dining room, there are eight plates on the table, but in the following shot one of them disappears.
- When Finn is accusing Princess Bubblegum of trying to avert suspicion by blaming Lumpy Space Princess, Finn and Princess Bubblegum appear to be the same height.
- When Jake was stretching his arms to trap Princess Bubblegum, his sleeves stretched with it.
- When Finn, Jake and Lumpy Space Princess cover themselves to avoid the ghost, they are at the opposite end of the table.
- Right after Princess Bubblegum melts, the white and black colors in Jake's eyes get switched for a couple frames.
- When Lumpy Space Princess gets sucked into the painting she drops her phone but in the painting she gets her phone back.
- This might not be an error as the incidence is later revealed to have been faked.
- In the painting, Lumpy Space Princess is near the end of the line of the dogs, but when the camera focuses on her, she is next to the fox.
- When Finn and Jake are trying to get out to the door, there is one scene that Finn's gloves are gone.
- When BMO took pictures of Lumpy Space Princess, Princess Bubblegum, and Jake BMO processed four pictures when it only took three pictures.
- When Finn first arrives and is standing by the staircase, his eyes appear to be almost on the top of his head behind his mask (see picture).
- At one point, BMO's bear mask turns on it's own with BMO moving his body, which would be impossible since the mask is set up on BMO's flat front
- When BMO and Jake are checking out the basement, out of Finn's earshot, they act as if neither of them know what is going on, when they obviously do.
- This is made more strange when BMO is clearly taken by Lady Rainicorn, who Finn was not using as part of his plan, although later Finn says that he had planned on BMO being taken. This means that his original plan for taking BMO could not have gone ahead.
- This episode was scheduled as part of Cartoon Network's Halloween-themed programming.
- On the On Demand synopsis, it said the invite was from Count Creepystein, but in the episode, they do not know what the host's name is.
- One could tell Princess Bubblegum was a dummy because she never moves when Lady Rainicorn comes in as the ghost.
- The fact that LSP hit and pushed away Cinnamon Bun right before the envelope was found on his back implies that she placed the note there.