The story unfolds at Mossy Bottom Farm, where Shaun and Timmy (both played by Justin Fletcher) alongside Bitzer and The Farmer (both portrayed by John Sparkes), begin their day with enthusiasm for their chores. With Halloween approaching, the sheep craftily cultivate pumpkins, keeping them safe and hidden.
Things take a romantic turn when a new Vet (played by Nina Sosanya) visits the farm, capturing The Farmer’s heart. However, Bitzer and the sheep try to evade the Vet to avoid any medical interventions. Unfortunately, the Vet manages to locate them, dispensing their medicine, and it seems she reciprocates The Farmer's interest. She proposes a Halloween event, which coincidentally aligns with The Farmer's own secret plans to ask her, but just then, he's pulled away by his mischievous goat, leading to an unfortunate accident where he wrecks the flock's pumpkin harvest.
In a desperate bid to remedy the disaster, Shaun stumbles upon a science book and concocts a mad scientist-like potion, determined to quickly grow new pumpkins. After planting the seeds with the potion—which he unwittingly confused for hair growth formula—he accidentally returns the container to The Farmer, who is keen to use it to impress the Vet with a fresh head of hair. As the night progresses, The Farmer wakes to find an unexpected increase in hair growth, much to his surprise, and soon, the pumpkins start swelling uncontrollably.
Dressed in a comically inappropriate leather outfit, The Farmer attempts to meet with the Vet, but his hair seems to have a mind of its own and grows incessantly. At her practice, he struggles to conceal his burgeoning mane. With each mishap, including a comical encounter with cement that leaves him stuck and noisy, The Farmer’s presence becomes an alarming spectacle for the animals of Mossy Bottom.
Shaun and the flock, sensing trouble, take refuge in the bathroom while Bitzer finds a separate hiding place. When The Farmer returns, his hair a wild mess, Shaun discovers a pair of glasses that reveal The Farmer's identity, leading to a series of panic-laden adventures. The chaos escalates when The Farmer, amidst an attempt to remove his hair, inadvertently invokes fear town-wide. Confusion mounts as the Vet starts believing she is on the trail of Bigfoot, hoping to whisk the “beast” away.
In a twist of fate, as Bitzer uncovers The Farmer's glasses, misunderstandings arise until Shaun clears the air. When The Farmer finds himself inadvertently incapacitated, the Vet manages to load him onto a boat for his “treatment.” The sheep brave the docks to rescue him from his crate, but disappointment hits when they find it filled with fuzzy dolls instead, leaving them feeling defeated.
Upon returning to the farm, the sheep are met with yet another setback: the pumpkins have proliferated to enormous sizes, devastating parts of their home. After being ostracized, Shaun heads back to the cave and discovers The Farmer’s missed calls from the Vet. Inspired by a shared bond, Shaun crafts a visual message for the Vet, informing her that the “beast” is indeed The Farmer.
The Vet, recognizing the urgency, hops on a boat to find The Farmer, with Shaun stealthily tagging along. As they traverse the ocean, the Vet encounters other trapped animals who collectively assist her in rescuing The Farmer. With a twist of fate, they manage to liberate him, and everyone sails back to safety.
Back on the farm, Shaun rallies the flock to create an antidote for The Farmer. However, townsfolk, still on the lookout, swarm toward the windmill where the antidote is prepared, and even The Shopkeeper (portrayed by Paul Whitehouse) tries to intervene. With quick thinking, Timmy and the other sheep use a giant pumpkin to stage a distraction, making it appear as though it consumed The Farmer. Just as all hope seems lost, Shaun successfully administers the antidote, hilariously reverting The Farmer to his normal state.
In the end, The Farmer and the Vet celebrate their connection at the much-anticipated Halloween event, while Shaun, Bitzer, and their companions craft Jack-o-lanterns. The festivities conclude, but not without a hint of mystery, as a crow is seen drinking the leftover growth formula, pulsing with a troubling glow, setting the stage for future shenanigans.
