In a theater, B.O.B. [Seth Rogen] introduces the story with a playful parody of classic horror openings, offering a cheeky recap of Mutant Pumpkins from Outer Space and warning the audience that the night ahead may truly bring nightmares.
The scene shifts to the twist ending of the previous special. The Zombie Carrot charges at the camera, but a gate slams shut, halting its advance. Ginormica's father, Carl Murphy [Jeffrey Tambor], announces to the kids of the Modesto suburbs that a costume contest has begun and that the winner will receive their weight in candy. B.O.B., dressed as a pirate, pockets the candy for himself. Outside, the zombie carrot terrifies him, yet he mistakes it for a “child in a costume.” Assuming the carrot will win the contest, he tosses it inside, where it bites Carl and turns him into a zombie. Wendy Murphy [Julie White] watches from the doorway as the chaos erupts around the Murphy house.
The guests flee, and the carrot is blasted by Dr. Cockroach [James Horan]’s scanner. Carl snaps out of the zombie state, and Dr. Cockroach theorizes that the carrot was contaminated by the mutant pumpkins, with the curse only liftable by eliminating the infected carrot. Yet the carrot’s remains replicate into more zombie lookalikes, reinfecting Carl. Against Dr. Cockroach’s advice, B.O.B. uses the scanner to blast the carrots, and soon B.O.B., Dr. Cockroach, and The Missing Link [David Kaye] join forces as they are completely surrounded.
General Monger [Kiefer Sutherland] arrives but falls victim to the zombie carrots, forcing Dr. Cockroach, Link, and B.O.B. to retreat inside the house and barricade themselves. B.O.B.’s clumsy moves lead to Link getting infected, and the trio retreats to Susan’s old room, where a stray zombie carrot appears to infect B.O.B.; this seems to have no effect, though, and Dr. Cockroach realizes that B.O.B. is immune because he has no brain.
Dr. Cockroach is ambushed by the carrot, and in his final moments before zombification, he advises B.O.B. that the only way to save the day is for him to eat all the carrots. B.O.B. reveals that he developed a fear of carrots after being force-fed carrot puree by General Monger when he was younger. Reluctant to eat the vegetables, he escapes via the window to the roof, but upon seeing his zombified friends, he resolves to eat every zombie carrot.
B.O.B. devours every zombie carrot, turning himself obese in the process. He nearly prepares to eat a Nutty Buddy Butter Bar for dessert when the original zombie carrot reappears, and B.O.B. cannot chase it due to his heft. The carrot grabs the bar and deliberately rips it open to taunt him, provoking a determined chase in which B.O.B. ends up swallowing the bar whole as a “chocolate-covered carrot,” even as he mutters that it’s disgusting. With the invasion finally over, Link, Monger, and Dr. Cockroach snap out of their zombie states and crowd around B.O.B. in a triumphant group hug, which sends him into a wildly oversized orange cloud.
Back in the theater, B.O.B. closes with a cheeky reminder to eat your vegetables, or they might eat you. Yet his shadow, as he leaves, morphs into a zombie carrot, hinting that the threat may not be entirely gone.
