Professor Moliarty and his henchmen break into a radioactive vault and steal some Canardian (a radioactive material) to power his latest creation. Darkwing shows up to foil his plans. Moliarty hides in the vault as Darkwing subdues his henchmen. Moliarty sees a timid Spider that has been affected by the radiation and has grown to his size. He runs out and runs into Darkwing and tells him about the giant spider. Darkwing starts to attack the spider and gets bitten on the thumb. The spider apologizes and Darkwing gets mad. The spider then spits out a web and ties Darkwing and Launchpad up. Moliarty figures that he can use the gullible spider for his own evil purposes and takes him with him. Darkwing and Launchpad are able to get free and return to the house where Launchpad wraps up Darkwing's finger. Gosalyn and Honker come in and ask what happened. Darkwing explains and then all of a sudden he grows four more arms to the shock of everyone. Honker does some research and finds that the only way to cure Darkwing is to get some venom from the same type of spider that bit him. An alarm sounds that there is trouble at the mall. Darkwing tells Launchpad and the kids to find a spider and he goes to the mall. He finds Moliarty with the spider and henchmen stealing fan blades. All of a sudden, two of Darkwing's extra arms start fighting against him allowing the villains to escape. Darkwing follows them down the tunnel they used to get in the mall and once again his extra arms betray him and Moliarty kicks him down a pit. Darkwing lights a light and sees that he is surrounded by Moliarty's henchmen and one of them blows out the lighter. The next scene shows the henchmen walking away from a very flat Darkwing. Darkwing pops up and gets out of the tunnel and decides to use his new arms and become a different superhero named Arachnoduck. As he is driving through the city, he comes across a giant web the hard way and gets stuck. He notices that there are planes and helicopters that are stuck in the web and their propellers are missing. Moliarty shows up in his helicopter and Arachnoduck uses his webbing (that he did not realize that he had) and brings down the helicopter. He tries to capture Moliarty and the spider but his own webbing ends up trapping him instead. Meanwhile, Launchpad and the kids are in the sewers looking for the spider that they need and they find it and chase after it. Professor Moliarty reveals his latest creation and shows why he needed the canardium and all the blades. It is a giant wind machine design to wipe everything off of the surface of the planet. Only the underground dwellers will be safe. The spider wants to go and fly a kite and Moliarty gives him a lollipop and tells him to go away. Arachnoduck shows up and is immediately captured and Moliarty puts him in front of the wind machine to be the first victim. Moliarty turns on the machine and starts flying Arachnoduck like a kite. Launchpad and the kids show up in the Thunderquack, but the winds shred it to pieces and they are hanging on for dear life. Moliarty cuts the web that was holding Arachnoduck and one of the extra arms throws his gas gun, hitting one of the switches, which turns off the fans. Arachnoduck thinks that he has foiled Moliarty but actually when he turned off the machine it started to cause the canardium to overload and was about to explode. Arachnoduck rushes to try and stop it, but the spider stops him and goes to stop it himself. The spider does it and comes out and shrinks from the radiation. They get the antidote to reverse the effects, but Launchpad wants to wait a little while before they use it because Darkwing is so "handy" around the house now.
Home video releases
- Darkwing Duck: Volume 1
- This was one of four Darkwing Duck episodes that was banned after its initial airings (the other three being "Tiff of the Titans", "Bearskin Thug", and "Hot Spells").
- Arachno-Duck is a parody of Spider-Man. In fact, this episode is based on the Spider-Man storyline, "The Six Arms Saga", in which the web-slinger tried to eliminate his powers with a serum, but it enhanced his powers instead, causing him to grow four extra limbs.
- When the henchmen and Moliarty tunnel into the lab, the henchmen come out first and their noses are black but when one of them call to Moliarty to come on out, his nose is a pink/tan color then changes back to black after Moliarty comes up.
- When Darkwing captures Moliarty's henchmen in the lab, it shows one of the henchmen wearing Moliarty's glasses even though Moliarty comes running in with his on.
- When Professor Moliarty and his two henchmen break into the radioactive vault, one of the henchmen asks Moliarty why they have to wear the protective suits and one of the henchmen tells why in Moliarty's voice then is back to Moliarty in the next scene.
- How could Darkwing blame his sixth arm for breaking the plate that fell on his right when that arm is on his left?