On a stormy night, a man enters a mansion and begins a speech in front of a group of people. However, he is interrupted by a sword falling through the window with a note attached to it, which reads "Last will and testament of Agatha Grisley". Meanwhile, the Ghostbusters and Slimer are watching a TV show about a mystery, and Winston spoils the ending for the rest of them.
The Ghostbusters get a call, so they drive off, with Winston revealing that it's to the mansion of Agatha Grisley, who died two years prior and who wrote hard-to-solve mystery novels. When they get to the mansion, another sword flies into the door, and Egon gets a PKE reading from it.
They talk to the man from before, who reveals that strange things have been happening since he got the will: swords falling, things breaking, women and smoking guns appearing and disappearing, and objects floating. Peter then falls down a trapdoor that appeared out of nowhere.
They explore the cellar, but then Ray gets suspended from the ceiling by chains with his limbs chained together, and Winston gets tied to train tracks. Luckily, they get free, but then a suit of armour swings at Peter with an axe, just missing him. The other three find an unfinished book and speculate that Agatha Grisley is haunting the house because she wants her story finished, and is presenting scenes from her stories to encourage living people to finish her story.
Thus, they decide to search for clues and read the story to see what would make the most logical ending for it. Ray and Peter find a man flirting with a woman, who's turning him down and claiming to be spoken for. Then, her husband shows up, accuses his wife (Debbie) of cheating on him, and brandishes a mace. The mace breaks the floorboards, and Peter and Ray fall through.
Egon scans a bedroom, where Debbie, now dead, appears on the bed and the flirty man cries over her body and reveals she was poisoned. A doctor shows up, and Egon offers the flirty guy a glass of water, but it turns out that was where the poison was. Egon then hears organ music, but the flirty guy accuses him of trying to kill him. He points a ridiculously huge gun at Egon, but luckily, Egon falls through the hole created by the spilled poison.
Peter searches the kitchen and eats some fruit, when a chef appears and starts cooking stew, cutting the meat by beating it up. Then, the dead Debbie and crying man show up again. Then, Egon walks in, pursued by Debbie's widower, who's now carrying the giant gun, and points it at the crying guy. Ray and the guy who got the will show up, and the crying guy kicks the widower in the shin, making him drop the gun and it goes off (but not killing anyone).
Then, the chef threatens to blow everyone up with a bomb. The suspects start arguing, and Egon and Ray try to determine if the bomb is actually dangerous or not due to being a spectral manifestation. Then, Winston enters, and determines the most logical killer: the doctor (since Debbie had a lot of medical bills she refused to pay and she made fun of the doctor's car).
With the mystery solved, the characters disappear, Agatha Grisley's ghost appears and thanks Winston, and then she and the bomb disappear. Then, a note appears dedicating the story to the Ghostbusters. Later, they watch the mystery show again, with Winston gagged so he won't spoil the ending.
This episode provides examples of
- Black Comedy: A man tries to kill Egon, but with a gun so ludicrously big he has trouble carrying and using it.
- The Butler Did It: Conversed when the Ghostbusters (except Winston) are watching TV and say, "The butler!" when the guy is about to reveal the culprit.
- Cartoon Bomb: The bomb the chef wields is a black sphere with a fuse.
- Chubby Chef: Downplayed — the chef is muscular but also has a bit of a belly.
- Comically Missing the Point: When Ray and Peter fall through the floor, Debbie says, "What rude men! They didn't even say goodbye!".
- Covered in Gunge: The chef gets coated in jam at one point.
- Creepy Changing Painting: At one point, a painting's eyes can be seen eerily moving.
- Creepy Red Herring: The murderer was the doctor, but three other men were suspect due to creepy behaviour:
- One guy was okay with flirting with a married woman and tried to shoot Egon (though to be fair, he did think Egon had tried to poison him).
- The chef had an angry demeanour and at one point threatened to bomb everyone.
- The widower also had an angry demeanour, was mad at Debbie because he thought she cheated on him, and threatened various people with first the mace then the giant gun.
- Death Glare: Egon and Ray give Winston a dirty look for referencing the Agatha Grisley books at an inopportune time.
- Dramatic Thunder: This spooky, death-focused episode opens on some thunder.
- Fair Play Whodunit: In-Universe; the show the Ghostbusters and Slimer watch is able to have the mystery solved by the viewer, which Winston does to the others' annoyance.
- Food Fight: At one point, the widower and the French dude start throwing food at each other.
- Gratuitous French: The flirty guy calls Egon "mon ami".
- Haunted House: When Agatha Grisley dies, she begins haunting her mansion using scenarios from her book.
- Implied Death Threat: The chef's threat to bomb everyone is "If [Debbie] can't eat [her favourite food], nobody should, and nobody should do anything else either".
- Labcoat of Science and Medicine: The doctor wears a lab coat.
- Literal-Minded: When a doctor says, "There, there" to the flirty guy (to reassure him), he replies, "Where? Where?".
- Lost Food Grievance: Peter yells at Egon for crushing his tomato.
- Mad Doctor: The mystery ends with the doctor being the killer.
- Mistaken for Cheating: Debbie's boyfriend thinks she's cheating on him, when actually another man was flirting with her and she was trying to turn him down.
- Mistaken for Murderer:
- Because Egon didn't realise the glass of water was poisoned, but the flirty guy knew, he assumed that when Egon offered it to him, he knew it was poisoned and thus was trying to kill him.
- At one point, the widower thinks it was the French guy who killed Debbie.
- Mythology Gag: Peter says, "Maybe there's a ghost in the refrigerator. Won't be the first time," referencing the original movie, in which a demon was in Dana's fridge.
- No Celebrities Were Harmed: Agatha Grisley's name puns on Agatha Christie and like Christie, she wrote mystery novels.
- Not So Stoic: Justified — Egon behaves uncharacteristically fearfully at one point, but it is because someone is threatening to murder him.
- One-Liner: Winston's response to getting chained to train tracks is just that he doesn't want to be taller because he doesn't like basketball.
- Priceless Ming Vase: Played for Horror — one of the signs the mansion is haunted is a vase breaking for no apparent reason.
- Pun-Based Title: The title puns on "whodunit".
- Russian Reversal: Egon tells Winston to end the mystery before it ends them.
- Sexophone: When the man in the suit flirts with Debbie, saxophone music plays.
- Shout-Out: Winston references Murder on the Hoban Express, a parody of Murder on the Orient Express.
- Skewed Priorities: Peter almost gets killed with an axe, yet Winston's more interested in the unfinished book he found.
- Slapstick: Peter trips on a discarded trench coat.
- Speak in Unison:
- When Peter says, "To the hunt?", the rest say, "The hunt!" in unison.
- After the bomb threat, Peter, Ray, and Egon say in unison, "Read faster, Winston!".
- When the guy on TV says, "It was...", Peter, Ray, and Egon say, "The butler!" in unison.
- Unconventional Food Usage: Discussed when Peter half-jokingly checks a banana to make sure it wasn't the murder weapon.
- Unfinished Business: The reason Agatha came back as a ghost was because she didn't want to go to the afterlife until her story was finished.
