While attempting to stop Professor Moliarty from stealing highly radioactive Canardium, Darkwing gets bitten by a radioactive spider and mutates, growing four extra arms. Darkwing is therefore forced to temporarily become a new superhero, "Arachno-Duck", until an anti-venom can be obtained. Meanwhile, Moliarty manipulates the spider, named Webby, into helping him build a machine that will blow every surface-dweller off the face of the planet.
This episode contains examples of the following tropes:
- After Action Patch Up: After the initial fight is over, Launchpad fixes up Darkwing's spider bite back at the house. He goes somewhat overboard with the bandages.
- All Webbed Up: Webby is able to spit webbing from his mouth, which he does several times. Arachno-Duck discovers late in the episode that his mutation gave him the same ability - unfortunately, he runs out pretty quickly.
- Distant Reaction Shot: Upon discovering he's gained four more arms, Darkwing screams so loud that it can be heard on Saturn.
- Going Critical: Even though Darkwing (and Number Six) shut off Moliarty's machine, the mole reveals he merely shut off the output, causing its generator to start building energy to dangerous levels to the point of going nuclear.
- Green Rocks: Canardium is represented as green-coloured bars of an unidentified material, complete with Sickly Green Glow to emphasise it's radioactive nature.
- Hazmat Suit: Moliarty and his moles wear these when stealing the Canardium, as it's one of the most radioactive substances on the planet. Moliarty even explicitly states that the radiation would kill them if they weren't wearing them.
- Heroic Sacrifice: After realizing just how much harm Moliarty's generator will cause, Webby prevents everyone else from entering the generator to avoid radiation poisoning and goes in to shut it off himself, reasoning he needs to make up for being Moliarty's Unwitting Pawn. While the radiation doesn't necessarily kill him, it does revert him back to an ordinary spider.
- Hostile Weather: Moliarty's plan was to build a gigantic hurricane machine to blow away all creatures above ground.
- Impact Silhouette: Moliarty is so terrified by the sight of Webby that he smashes through the heavy iron door of the Canardium depositry, leaving one of these behind.
- Minion with an F in Evil: Webby becomes this to Moliarty. The mole is easily able to manipulate Webby, but the spider is so innocent that Moliarty has to hide his villainous nature from him, which starts to drive him up the wall.
- Multi-Armed and Dangerous: As a result of being bitten by Webby, Darkwing ends up getting four extra arms. Unfortunately, they tend to be more of a hinderance than a help.
- Mundane Utility: At the end of the episode, Darkwing's extra arms come in handy...for doing the dishes.Darkwing: A fine pass-time for a crime-fighter, exploited by my own family!
- Nice Guy: Webby is this to a fault, which is what allows Moliarty to manipulate him. He only even bit Darkwing because the latter assumed he was dangerous and Webby was defending himself.
- Nuclear Mutant: Webby, a spider grown to human size by Canardium radiation. Darkwing himself becomes one of these after Webby bites him in self-defense.
- Obliviously Evil: Webby comes across as a naive, simple-minded creature, who is initially unaware that his actions will have dire consequences and that he's being exploited by Moliarty. Upon realizing what Moliarty's true intentions are, he decides he Must Make Amends by shutting off the radioactive generator before it melts down.
- Organ Autonomy: Darkwing has a hard time controlling his extra arms, especially Number Six, which seems to have a mind of its own.
- Spider-Man Send-Up: Drake Mallard as Arachno-Duck is one of these. Aside from sharing a similar "bitten by a radioactive spider" origin, Arachno-Duck wears a red-and-black costume with a yellow spider-web design on the chest, similar to Spider-Woman, and can shoot organic webbing... except he does it from his beak and not wrist-mounted launchers.
- Villainous Plan Inertia: Darkwing/Arachno-Duck is able to shut off Moliarty's Wind-O-Matic, but not the Canardium generator, which starts to go critical as it overloads. Moliarty's gleeful reaction indicates that this is just fine by him, as St. Canard is doomed and he wins either way.
- Why Did It Have to Be Snakes?: A terrified Moliarty calls a truce and runs for Darkwing's help at the sight of the giant mutant spider. He gets over it quickly, however, when he sees Webby's abilities and how useful they could be to him.
