The Ghostbusters are called to a posh house belonging to a woman named Mrs. Campbell, with Slimer accompanying them. Mrs. Campbell claims that a ghost made a mess of her house and scared her dog Pieface, but when the Ghostbusters scan, they find no sign of anything supernatural. Slimer, meanwhile, is hungry as per usual and tries to eat Pieface's food. When Pieface growls at Slimer, he goes into the kitchen, where he finds two turkeys. He's about to eat them, when one of them comes to life and then morphs into a copy of Pieface.
The Ghostbusters get into their car to leave, but unbeknown to them, the fake Pieface exits the house, turns into a copy of the Ghostbusters logo, and jumps onto the actual logo. When they get to the firehouse, the shapeshifting creature goes upstairs, while Winston starts repairing Ecto-1, and Ray and Egon go to work building an interdimensional portal. Peter asks Slimer if he wants to help him do the laundry, and Slimer reluctantly agrees, but only if he can eat a snack first.
He goes to make himself a burger, but then sees a bigger burger, which comes to life and runs downstairs. Slimer tells Peter about what happened, but Peter doesn't believe Slimer, and then the creature morphs into Peter. The fake Peter walks over to Winston and spills oil on him, angering Winston so much he chases him. Then, the creature turns into a lamp, while Winston confuses the real Peter (who's now playing "laundry ball") by yelling at him.
The creature then turns into Janine and visits Egon and Ray in the lab, who explain that they're building a portal to the Netherworld in case the containment unit runs out of space. The fake Janine is scared of a terror dog that can be seen in the portal, but the guys explain that it has a safety switch. Then, she exits the lab and is seen by the real Janine, who screams. She tells the guys that she saw "herself" walking out of the lab, but they think she's hallucinating and make her lie down.
Then, the creature changes into Egon, enters the lab, and turns off the portal's safety switch, claiming he did it "just because". Ray fixes it, then confronts Egon in the living room, but he denies that he'd ever switch off the portal's safety or say, "Just because". Meanwhile, Peter and Winston are still arguing over the oil. When a chair comes to life, Egon realises that there is a "metamorph" (shapeshifter) in the firehouse, which Ray dubs the Copycat. Egon locks the firehouse so the Copycat can't escape, then they split up and search.
Peter searches the bedroom and accidentally shoots Ray's radio, Ray and Winston search the kitchen and find the Copycat disguised as cheese, but it runs away, then Egon searches the lab and finds the Copycat disguised as a brain in a jar. He fires at it, but accidentally shoots the portal, sending the terror dog into the room. Egon sets off an alarm, summoning the others, and they send the terror dog back through the portal. Then, the Copycat turns into Captain Steel (a character from Ray's comic books) and starts firing at the Ghostbusters with its laser vision.
They go downstairs, where the Copycat turns from a lamp into a laser and tries to attack the Ghostbusters, but misses. Ray, noticing that metamorphs, this one in particular, are playful, decides to have Slimer trick the Copycat into assuming his form under the guise of a game. That way, they'd be able to trap the Copycat as they would a ghost. He does, and (with Peter holding up a chocolate bar so they can tell which is the real Slimer), they trap the Copycat.
This episode provides examples of
- Admiring the Abomination: Ray makes baby talk to the terror dog when it's contained behind the portal.
- Ambiguous Gender: It's never made clear if Pieface is a male or female dog.
- Animate Inanimate Object: The Copycat changes into a living Ghostbusters logo, lamp, table, brain in a jar, hula girl lamp, laser beam, and footstool.
- Anthropomorphic Food: The Copycat takes the form of a walking roast turkey, burger, and block of cheese.
- Baby Talk: Mrs. Campbell affectionately babbles to Pieface, then later, Ray does the same to a terror dog.
- Bannister Slide: When the Copycat is taking the form of Pieface, it slides down a railing.
- Blowing a Raspberry: When the Copycat is taking Slimer's form, the two of them blow raspberries at each other.
- Brain in a Jar: At one point, the Copycat disguises itself as a jarred brain in Egon's lab. Upon realizing he only had one, Egon knew that one of them was the Copycat.
- The Compliance Game: In order to trap the Copycat, it has to take the form of a ghost, so to get it to do so, Slimer must trick it into taking his form as a "game".
- Dog Stereotype: Pieface is a poodle and its owner is wealthy.
- Ectoplasm: Slimer can sometimes be seen leaving green gunk, and Egon claims that the shutters don't let ectoplasm through.
- Evil Laugh: The Copycat often laughs maniacally.
- Eye Beams: As Captain Steel, the Copycat has laser vision.
- Hellhound: A terror dog (an evil, doglike demon) features.
- The Hyena: The Copycat laughs a lot whenever it has a form with a mouth.
- I Will Show You X!: When the Copycat, pretending to be Peter, claims that spilling oil on Winston was "funny", Winston yells, "I'll show you funny!".
- Joke and Receive: When Slimer says he tried to eat a hamburger, Peter says, "And what did it do, grow legs and run away?". That's exactly what it did.
- Laughably Evil: The Copycat tries to kill the Ghostbusters with lasers and opens a portal to the Netherworld, but it's also quite silly, playing a prank on Peter and Winston and taking the form of walking objects.
- Let's Split Up, Gang!: When searching for the Copycat, Slimer and Janine search the reception area, Peter searches the bedroom, Ray and Winston search the kitchen, and Egon searches the lab.
- Lost Food Grievance: Pieface growls at Slimer for eating the dog food.
- Mythology Gag: Slimer is called a "spud" several times, which references him being called one in the movie, and a terror dog (who Dana and Louis turned into in the movie) features.
- Nice Kitty...: Egon tells the terror dog to "be a good dog".
- Oh, Crap!: Egon realises it isn't his lucky day when he accidentally frees the terror dog.
- O.O.C. Is Serious Business: When the Copycat is pretending to be Egon and laughs, Ray says in a shocked voice, "Egon, what are you doing!?", then is even more shocked when he turns the portal's safety switch off "just because". The real Egon is The Stoic and always has a reason for what he does.
- Platonic Kissing: At the end, Slimer kisses Peter.
- Scooby Stack: When Janine says she saw "herself", she, Egon, and Ray appear stacked in the doorway.
- Slapstick: Ray and Winston end up bumbling about a bit when trying to catch the Copycat, who's disguised as cheese.
- Snooty Haute Cuisine: Parodied when Peter jokes that Mrs. Campbell's house is so fancy, its mice must eat French cheese.
- Spot the Imposter: Peter can tell which is the real Slimer by seeing which one wants the candy bar he holds up.
- Stock Animal Diet:
- Joked about when Peter jokes that the mice in Mrs. Campbell's house must eat French cheese because the house is so fancy.
- Played with when a mouse is seen with the Copycat disguised as cheese.
- That Makes Me Feel Angry: Mrs. Campbell says, "I'm furious!" when describing the mess the Copycat made.
- Theme Music Power-Up: The main theme plays when the Ghostbusters trap the Copycat.
- Uncatty Resemblance: Pieface's fur on top resembles Mrs. Campbell's hair.
- Voice Changeling: The Copycat can perfectly imitate the voice of whomever it's impersonating.
- Voluntary Shapeshifting: The Copycat can change form at will.
- Weirder Than Usual: When Ray finds out about the Copycat, he says, "No wonder everyone's been acting so weird!". Winston replies, "You mean weirder than normal?".
- Wingding Eyes: Slimer gets candy bars in his eyes when he sees the candy bar.
