The episode begins with Gerald, Stuart, and Roger arriving at Ryan's house to pick him up. All four men were dressed as witches, speaking of preparing for a special annual Halloween event. The scene shifts to Randy at the Marsh Residence who argues with Sharon about allowing him to participate in the special event with the other guys. Sharon insists that he come with her to take Stan to the pumpkin patch, where a carnival was being held. Randy ignores Sharon, and goes to hang out with the guys anyway.
Meanwhile, at the Turner Residence, Cartman waits impatiently for Heidi to get ready to leave with him to the pumpkin patch, eager to arrive on time. Heidi takes way longer than Cartman suspected and he frets about being late to the pumpkin patch. While hurrying her out the door, Heidi’s parents, Mr. Turner, and Mrs. Turner request a photo of all of them together, showing support for Heidi and her relationship with Cartman. By the time Cartman and Heidi got to the pumpkin patch it was dark out and all of their friends were leaving. Most of the attractions were taken down, and there were very few pumpkins left that matched Cartman's tastes.
Randy and the other men of South Park were partying at the top of a mountain, getting high and drinking a lot of beer. The outsider of the group, Chip Duncan, introduced the guys to an ancient spell book, from which he recited a hex, and accidentally turned himself into an actual witch. Chip's body stiffened and bent, his eyes glowed bright under his now larger forehead. He had a smile plastered across his face, and took off on a broomstick. The other guys were too buzzed to fully grasp the situation but were still aware of the situation enough to recognize the trouble that lied ahead of them. Chip flew immediately toward the pumpkin patch just as Cartman and Heidi were getting ready to go inside, forcing them to leave, and further irritating Cartman. Children were still leaving the carnival, and many of them, including Peter Mullen were tormented and kidnapped by Chip.
The following day on the news at the Marsh Residence, Chip is reported as a witch abducting children. Sharon sees this, and argues with Randy about it, disappointed that he wasn’t taking the situation more seriously. At the elementary school cafeteria, Cartman is staring intimidatingly at Heidi from the table across from her. As he stared at her, the other boys discuss about how they can get rid of the witch. Instead of telling her how he felt about how long she can take to get ready for things, Cartman overhears the boys' plans to get rid of the witch at the last minute and assumes they were talking about Heidi. Cartman unknowingly agrees, and instead devises a plan to get rid of Heidi.
Randy once again ignores Sharon's warnings and goes to hang out with the other guys for a second time. The guys reach the top of the mountain at the site where they usually party. They soon discover that it had been closed, and believe that Chip is making people bias towards other "good boy" witches. He believed the town was having a "witch pursuit thing", and he and the other guys grow irritated with Chip for spoiling the towns opinions of witches. In a poor attempt at fixing the towns opinions of witches, the dads of South Park perform at the elementary school during an assembly, and afterwards they give a speech. When the boys interrupt them, they call them "sons a witches", embarrassing them.
Kyle, Stan, and Kenny go over their plans to get rid of the witch. Cartman interrupts, and makes his own suggestions, believing they were actually helping him get rid of Heidi. Kyle snaps at Cartman for his mistreatment of Heidi, but Cartman blames this on her poor time management skills. Cartman leaves the boys. Instead of telling her how he felt about how long she takes to get ready for things, Cartman brings his plan into action. Later in the episode, Stephen talks to Randy about how guilty he feels about being a witch, and how he planned to confess to his wife, Linda that he and the other guys drunkenly performed fake hexes on each of their respective wives. Randy pretends to be on board with Stephen's intentions but as soon as he leaves he calls the other guys to help him sacrifice Stephen to the devil.
Cartman dismisses Kyle's advice, and later he convinces Heidi to dress up as Gretel, while dressing himself up as Hansel. Immediately after he takes Heidi into the woods with him at night, and ultimately ditches her in the woods to die, and gets her kidnapped by the witch. However, shortly after he flees to the police at the Park County Police Station to inform them of Heidi’s kidnapping, supposedly deeply regretting it, but also while trying to avoid being labeled as a suspect.
Confused about what role their dads might have played in releasing the witch in South Park, the kids have Butters call President Garrison over the phone. After unintentionally bothering him with questions, Butters asks Mr. Garrison to explain to him and the boys about the annual Halloween witch event their father's were holding, learning from an old photo that Garrison used to be involved before is candidacy. Garrison comes to South Park to help them. Meanwhile, Randy and the other dads surround Stephen in front of a Ross Dress for Less, admitting to finding the store desolate and spooky. They prepare to sacrifice Stephen but are confronted by Chip. Chip shows them a magical bottomless sack, a 'bag of holding', where he reveals to them is where he stored all the children he kidnapped, including his most recent victim, Heidi. The guys ask him to stop behaving so crudely because he is giving other "good boy witches" a bad reputation.
At the end of the episode, Garrison appears, with the boys close by. He orders Chip to stop tormenting people. Chip refuses to comply with him. Bored and sick of negotiating with him, Garrison has a laser fired from a nearby satellite with coordinates set directly onto Chip, instantly killing him. The police arrive and recover Chip’s bag. They set all the children free, except Heidi, who needed a little more time to prepare for her own rescue, which of course annoys Cartman.
AV Club gave "Sons A Witches" a "B+" rating saying: "This year’s Halloween episode doesn’t go as large-scale with its parody as something like the show’s Scooby-Doo bid, “Korn’s Groovy Pirate Adventure.” But it does occupy the same comedic-horror space and suburban-gothic menace as specials such as The Halloween Tree and Garfield’s Halloween Adventure. The plot involves missing children; trees straight out of a dark fairytale scratch the sky. There’s plenty of autumnal atmosphere to go around. Even the main antagonist—a direct spoof of The Green Goblin from Sam Raimi’s Spider-Man—manages to be somewhat frightening, if only for being an actual witch who actually wants to harm little kids."
IGN gave "Sons A Witches" a "8.3" rating saying: "South Park continues to establish a winning formula this season, focusing on more standalone storylines that still pull from current headlines in clever ways. "Sons a Witches" works as both a commentary on the Harvey Weinstein scandal and its fallout and fun, simple tale of Randy and his friends being stupid. The escalating Cartman/Heidi storyline and the surprise return of President Garrison didn't hurt."