A fungus mutant spreads through the sewers, causing April and the Turtles to experience their worst fears; Leonardo must face his greatest fear: failing his team.
This episode provides examples of:
- An Aesop: Splinter asks Leonardo at the end if he's mastered his fear of failing his brothers. He admits he has not, only that he's learned not to let it consume him, making a much more realistic take on the Face Your Fears trope. Not every fear can be conquered quickly, but you can indeed not let it control you. Splinter is proud of Leonardo for this answer, and says that proves he is a good leader.
- Angry Fist-Shake: One of the mushrooms before it withers.
- Art Shift: During Mikey's spore-induced hallucinations, the graphics around him begin to resemble a comic book (more so than usual), complete with speech bubbles and Written Sound Effects.
- Bait-and-Switch: The construction worker running off from squirrel monsters makes you think this will be a Sequel Episode to Attack of the Squirrelanoids. You find out that he was running away from something else just a few moments later.
- Break the Cutie: April is the first to fall under the spores' effects and spends most of the episode hallucinating her father and other bats are trying to eat her.
- Combat Tentacles: The giant mushroom is able to summon multiple stipes and use them as whips.
- Continuity Nod: All of the characters' fears nod to past episodes. Raph's fear of cockroaches and Spy Roach in particular, Casey's fear of rats, Mikey's fear of the Squirrelanoids, Donnie's fear of April rejecting him for ruining her life/being a horrible mutant, April's fear of her mutated father, and Leo's fear of failing as a leader.
- The Squirrelinoids get another reference near the beginning when a (presumably fear-spore addled) construction worker runs by screaming about mutant squirrels.
- Darker and Edgier: One of the more disturbing episodes. The way most everyone was taken down so easily and the implications of fear-inducing spores over the city is like a Scarecrow plotline straight out of a Batman story.
- Disney Acid Sequence: After Leo gets sprayed with spore and starts seeing his fear, we're all treated to a random scene of all the mushrooms dancing to the "Dance of the Sugarplum Fairies."
- Dude, Not Funny!: When Mikey brings pizza for everyone at the end of the episode, absolutely nobody is amused when he reveals it has mushrooms on it, after everything they went through with the mushroom mutant.
- Dying Declaration of Hate: A nonverbal example. As the others go to check on Leo, one of the withering Mushroom Men lets out an Angrish squeak at them as they pass by while doing an Angry Fist-Shake before it finally expires.
- Extra Eyes: The Giant mushroom has these with the smaller eyes serving as Abnormal Ammo.
- Festering Fungus: Multiplying. Mushroom. Mutants.
- Fungus Humongous: As the title implies, the episode's main mutant is fungus-based. And is humongous in size.
- Headbutting Heroes: As the fans predicted, Donnie and Casey have officially started this.
- MushroomMen: The minions of the giant mushroom. At first, they seem to be quite adorable. That is until they start attacking the heroes and spraying them with their spores.
- Mushroom Samba: Apparently the mushroom's spores cause its victims to see their greatest fears.
- No Ontological Inertia: The moment the mutant of the day goes down, the entire team recovers from their individual Mushroom Sambas.
- Shout-Out:
- As could be expected when fighting knee-high mushrooms, Mikey takes on the mushroom men Super Mario Bros. style, complete with a temporary Art Shift to pixel art.
- The mushrooms themselves appear to be one big shout-out to the suite
from Fantasia what with their design and their dance that not only uses the music but also similar choreography. - "What is your malfunction?"
- Vomit Discretion Shot: Raph pukes when Mikey tries to hug him, viewing him as a giant cockroach hugging him.
- Weakened by the Light: The mushrooms die when exposed to sunlight.
- What Happened to the Mouse?: So... what happened to the sewer worker guy who thought he saw a Squirrelanoid? He just disappears and nobody seems to care about him.
- "What Do They Fear?" Episode: Most of the main characters' greatest fears are being used against them via the mutant spores.
- April hallucinates that she is being attacked by bats, ranging from regular bats, to thinking that her friends are giant bats to seeing her father post-mutation.
- Casey thinks that he is being swarmed by entire rivers of rats.
- Raphael thinks that his brothers are giant cockroaches.
- Micheangelo vomits up squirrels and thinks he is being chased by Squirrelanoids from "Invasion of the Squirrelanoids."
- When Donatello tries to comfort a frightened April, she then proceeds to mock and insult him, her eyes white and teeth sharp. He even briefly sees a group of love-struck Aprils fawning over Casey.
- Leonardo's recurring nightmare is made real; that his friends and family are being dragged off into the dark and he is powerless to help them. He is also briefly put down by Captain Ryan from Space Heroes and hallucinates that the small mushroom mutants are dancing to Sugar Plum Fairy (
though that may have actually happened...).
