Arnold's Halloween is a special half-hour episode of Hey Arnold!.
The episode opens on Arnold and Gerald watching the "Z-Files" at the Sunset Arms, and the episode ending on a massive cliff-hanger. Grandpa enters, and discusses with the boarders who will dress as what at their Halloween party this year, leading to Mr. Hyunh and Ernie arguing over who will be Frankenstein (The monster) Arnold asks his Grandpa if he and Gerald can help out with the party this year, but Grandpa tells him that they're just kids. Wanting to prove they can scare the adults, Arnold and Gerald formulate a plan to scare the boarders during the party.
Meanwhile, Big Bob is playing cards and watching UFO Tonight with Mr. Green and Harvey, and is telling them his alien abduction story, which they dismiss, and Helga has evidently heard many times before. However, it gives her the idea of having the P.S. 118 fourth-grade class dress up as aliens for their group Halloween costumes.
She reveals the plan the next day and, despite other ideas being thrown around, has the final say. Helga asks Arnold and Gerald if they want to join in. Arnold tells her they have other plans, but also tells the group to stop by his house's party. He tells Gerald it will be perfect for their prank, but neglects to tell the others what is planned.
On Halloween night, Helga is dressed in her alien costume, and attempts to show it to Bob, who is preoccupied with The Wheel. Meanwhile, the boarders and their guests are enjoying the party, although the trick-or-treaters aren't, as Grandpa is handing out worms and cockroaches. On the roof, Arnold and Gerald are setting up a phonograph to change Gerald's voice, and various props for visual and sound effects, in order to simulate an alien invasion.
They intercept the boarding house radio, and scare the living daylights out of all inside, convincing them that aliens are invading. All the while, Stinky is prepared with lights set up on the water tower (in order to make it appear as a UFO), and turns them on when he gets a signal from Arnold and Gerald. Unknown to them, an operative of UFO Tonight picks up the signal in a van, and reports back to the host, Douglas Cain, who airs it live on television. He also gets footage of the kids coming to the door. Grandpa tries to dismiss it as a prank, but once Helga and crew arrive, the boarders think they are real aliens and chase them with weapons. All of this is caught on camewra and the city goes into a panic.
As the chaos ensues, Grandpa is packing the Packard to leave town, when Arnold and Gerald arrive, unaware of what is happening. The two explain their whole plan and Grandpa is impressed with their work. However, when Grandpa asks how they got Douglas Cain involved, Arnold and Gerald realize what happened and ask what happened to their classmates. Just then they see them being chased by a mob and they decide that they have to save the kids.
Elsewhere, Bob gears up to take down the aliens, as does Grandma, while the kids try to explain they they're just kids,. Unfortunately, Harold, who was tasked with buying makeup, purchased a kind they couldn't remove. They're chased to the water tower, and Bob, Grandpa, Arnold and Gerald, and the UFO Tonight crew are headed in that direction as well. Arnold and Gerald try to explain the prank, but the people in the street are either unable or unwilling to listen to them.
Bob, accompanied by Marty and Harvey, encounter Principal Wartz, who is prepared with candies for the aliens, and they pull him into the Hummer. When the kids reach the water tower at the top of the hill, they run into Stinky and tell him what's going on. Stinky inadvertantly reveals Arnold's plan and a furious Helga attacks him. The other groups arrive, and Bob sets up an explosive catapult, and goes to confront the "alien leader" (Helga), and attempts to wring her neck. Arnold tries to stop him, and Harvey fires the catapult into the water tower, which bursts and washes off the makeup. Bob apologises to his daughter and gets fed up with Cain's broadcasting. As they all leave, Arnold tells Grandpa he'll never do a prank like that again, but Grandpa is proud of Arnold's prank. Grandma turns the water tower lights back on, and cackling into the night as she wishes everyone a Happy Halloween.
- This episode is based on Orson Welles' 1938 radio broadcast The War of the Worlds, which also sent many people into a panic.
- A "Welles Ridge" is mentioned in the episode. Obviously, it is a reference to Orson Welles, who's 1938 radio broadcast is parodied in this episode.
- This is the second episode where Helga yells out "ARRRNOOOOLLD!", the first time was in "Downtown as Fruits." Technically it would also be the third as the first time she yelled out Arnold's name was in the Pilot episode.
- Maurice LaMarche, who voices Big Bob and Douglas Cain, voiced Orson Welles in The Critic episodes "Eyes on the Prize" and "Frankie and Ellie Get Lost." He also dubbed over Vincent D'Onofrio's role as Orson Welles in "Ed Wood". The character of the Brain from Pinky and the Brain is also based around LaMarche's Welles impression.
- The costume and makeup selected by the kids for their hoax are similar to that of the Talosians from Star Trek: The Cage (#1.0) (1966).
- The wheel on the show Big Bob was watching had spaces marked OOPS!, Bye-Bye, No Cigar, New Car, See Ya, Winner, Whole Enchilada, Sucker, Loser, Spin Again, and S.O.L. (the latter of which is an acronym for the phrase "sorta outta luck").
- Big Bob, when perusing the sports section of his newspaper, mentions "Green Bay". This is probably a reference to the Green Bay Packers football team.
- All of the credits (except in the show's opening) are orange instead of the usual yellow.
- This episode aired 4 days before the actual day of Halloween on October 31st.
- This is the only special half-hour episode of Season 2.
- This is one of the few episodes where Helga shows no indication of her love for Arnold.
- In the Shout Factory DVD release of Season 2, the episode is listed as "Operation: Arnold's Halloween", as well as in the Hey Arnold! The Complete Series DVD set.
- Normally, Grandma gets her holidays confused, but here, she knows it is Halloween.