The story follows a young girl named Zoe, whose mother died when she was a baby, and who is trying to navigate life with a father who has just lost his job at an ice cream factory. They share a council flat with Zoe and her father, while her large stepmother, Sheila, Sheridan Smith as Sheila, relies on scant government benefits. Sheila is portrayed as lazy and self-centered, obsessed with prawn cocktail crisps and cigarettes, even to the point of asking Zoe to pick her nose. Zoe endures daily taunting from a classmate named Tina Trotts, a problem that colors much of her school life.
A few years earlier, Zoe’s father surprised her with a hamster named Gingernut for her birthday. When Sheila arrives on the scene, she wants Gingernut out, and Zoe grows close to the tiny pet, teaching it tricks and dreaming of becoming a famous animal trainer. Tragically, Gingernut dies under suspicious circumstances, a wound that lingers over Zoe’s world. Months later, Zoe discovers a baby rat that has somehow sneaked into her room, and she finds that this new creature is intelligent and responsive to music, capable of dancing along. She teaches the rat tricks just as she did with Gingernut, naming it Armitage after a plumbing company she spots on a toilet bowl. Because Sheila hates rodents, Zoe hides Armitage in her blazer and tries to keep him secret, but her headmaster, and her history teacher (Miss Midge), discover the pet and threaten suspension because pets aren’t allowed at school.
Seeking help, Zoe goes to the local newsagents to talk to Raj, who is shocked by Zoe’s suspension and wary of Armitage. He advises Zoe to set the rat free, a suggestion she initially resists, but Armitage’s persistence convinces her that he wants to stay with her. Sheila’s discovery of Zoe’s suspension escalates tensions at home, and she pressures Zoe to hand Armitage over to Burt, the blind burger seller who presents himself as a pest-control man. Zoe refuses, but Burt’s sleuthing eventually catches up with her, leading to Zoe being locked in her room. In a bid to escape, Zoe attempts to dig a hole through the wall into the neighboring flat, only to stumble into Tina Trotts’s bedroom and flee.
Caught in the middle of this mess, Zoe spies on Burt and learns the shocking truth behind his operation: his burgers are made from rats he has captured, and his ketchup is brewed from cockroaches. Burt explains his bitter history with rodents: while working in a laboratory, a rat bite caused acid to splash into his eyes, leaving him with extremely keen senses of smell. He uses this sense to track Zoe and Armitage, and when confronted, he tries to I turn the situation violent, attempting to run Zoe over and turn her into another recipe. Zoe escapes once again.
Desperate for help, Zoe and her father return to The Executioner & Axe to gather courage. They venture to Burt’s warehouse where the rats and the burger-making machine reside, freeing all the captive rodents because they cannot identify Armitage among them. Burt and Sheila arrive, dragging Armitage along, threatening to throw Zoe and her father into the machine. Sheila then reveals Gingernut’s death, poisoned by rat poison in Gingernut’s food. As the plan to sacrifice Zoe and her father looks imminent, Sheila’s weight collapses a ladder, sending both Burt and Sheila to the machine. They are transformed into burgers, while Armitage survives because Burt’s reflexes save him, and Zoe’s father clings to the machine with one hand. In a final, grim twist, the rats feast on the burgers intended for Burt and Sheila.
With Armitage in tow, Zoe and her father escape in Burt’s van, which he later converts into an ice cream van and renames Armitage’s Ices. The venture becomes a hit with local schoolchildren, helping to chart a fresh start for the family. In a parallel turn of events, Tina reveals she has been bullied by her own father, and she apologizes to Zoe for the way she treated her earlier. The two form a new, fragile friendship. The next day, Zoe’s suspension is lifted, and she and Armitage perform at the school talent show, joined by Tina, bringing a sense of closure and renewed hope to Zoe’s world.
