iScream On Halloween is the first Halloween episode of iCarly.
It's Halloween and a letter addressed to Apartment 13B has been accidentally delivered to Carly and Spencer, so Carly heads downstairs to give the letter to Lewbert. On her way down there, she finds Freddie dressed in the Halloween costume that his mom made—a witch's cloak, broom, and a wart.
Downstairs, they give the letter to Lewbert who (after yelling at Freddie for making fun of his own wart) tells them no one's lived in Apartment 13B for 15 years because it's haunted. Mrs. Halberstadt, a longtime resident of the building, enters the foyer and Carly and Freddie try to ask her about Apartment 13B, but she can't hear them. Carly and Sam decide to do a special Halloween episode of iCarly from Apartment 13B.
The gang prepare for their webshow, with Carly donning a queen bee costume, and Freddie dressing up as a witch (because his mom made him wear it) but Sam doesn't dress up. Sam picks the lock of 13B and they go in, and start broadcasting, promising to stay on the air all night, no matter what. The promise lasts only a few minutes before the lights go out, glass starts to shatter, and an angry voice begins yelling "Get out!" The girls scream and Freddie tries opening the door to the apartment, but the knob comes off. Sam tries using Freddie as a battering ram.
Carly, Sam and Freddie take refuge in a closet, but are terrified by a disembodied clown's head they find. They head to the kitchen and Sam finds her hands covered in what appears to be blood.
Soon, the door opens and a middle-aged man comes into the room, while Mrs. Halberstadt, his mother, comes out of the back room. The gang soon discover that Mrs. Halberstadt is completely deaf without her hearing aid turned on. The group works out that the "haunting experiences" are little more than Mrs. Haberstadt yelling at a bug ("Get out"), a spilled bottle of hair dye (blood on Sam's hands), a clown head telephone, a short-circuiting light switch and a kitten named Abigail (knocking over some glassware and scratching their legs). As for why they were told the apartment had been abandoned, Lewbert comes by to say it's because he's a jerk.
Spencer plans to spend the evening carving a pumpkin using a design so scary even the blueprints scare him and Carly. Spencer's pumpkin arrives—and it ends up being so big he needs to use a chainsaw to carve it. Spencer continues to work on the giant pumpkin, but is interrupted by trick-or-treaters. He realizes, too late, that he's forgotten to buy candy, so he starts passing out other items, including diet root beer, ice cubes, macaroni and cheese, and even a set of jumper cables. He finds himself besieged by an angry mob of 15 children who storm the apartment demanding candy. He takes cover inside the giant pumpkin, but he's discovered and the kids take the pumpkin down the elevator and roll him out of the building into traffic. It is revealed in iMeet the First Lady that a fish truck then pushed him into Bell Harbour. The pumpkin didn't float, it sank like a rock. He manages to escape and returns to the apartment in time to help Carly and Sam end their Halloween broadcast with an important piece of advice: "Never forget to buy candy on Halloween!"