As the Whos of Whoville eagerly prepare for their cherished Christmas celebrations, a bitter and misanthropic creature known as the Grinch, who lives in a cave on Mount Crumpit, harbors a deep hatred for the holiday and plots to ruin it. Six-year-old Cindy Lou Who, feeling disillusioned by the community's materialistic fixation on Christmas, crosses paths with the Grinch at the Whoville post office. In a surprising turn of events, she is saved by him after getting caught in a mail-sorting machine.
Driven by her curiosity towards the Grinch's animosity towards Christmas, Cindy learns from the Whoville townsfolk that he was abandoned as an infant and subsequently adopted by two elderly sisters in their town. During his school years, he developed a crush on his classmate, Martha May Whovier. Unfortunately, their budding feelings are disrupted by Augustus May Who, who, filled with jealousy over Martha's attention towards the Grinch, relentlessly bullies him for his unkempt appearance. In an attempt to win Martha's affection, the Grinch makes the ill-fated decision to shave his face, inadvertently causing injuries. After enduring ridicule from his classmates during a Christmas celebration, he breaks down, declaring his aversion to Christmas. Overwhelmed with shame and humiliation, the young Grinch escapes to Mount Crumpit, vowing never to return.
In a bid to include the Grinch in the festive spirit, Cindy boldly nominates him as the town's Holiday Cheermeister, causing a stir among the Whos and provoking the ire of the now-Mayor Augustus May Who. Hesitantly, the Grinch consents to participate, hoping for a chance to confront his childhood adversary and rekindle his connection with Martha. The award ceremony offers him a glimpse of joy as he dunks and engages in various merriments, but his mood takes a downturn when May Who presents him with an electric razor — a jarring reminder of his troubled past. To add insult to injury, Augustus proposes to Martha in front of the gathered crowd, presenting her with an extravagant car as a token of his affection. Infuriated, the Grinch lashes out at the Whos, accusing them of prioritizing materialism over the true essence of Christmas. He shaves the mayor’s head, sets the town's Christmas tree ablaze, and embarks on a wild spree of destruction before retreating to the safety of his cave.
Yet, when he discovers that the Whos intend to celebrate Christmas despite the setback, gathering with an unbreakable spirit, the Grinch concocts a devious scheme to steal their presents, decorations, and festive food while they slumber, aiming to extinguish their holiday cheer for good. Disguising himself as Santa Claus and dressing his loyal dog, Max, as a reindeer, the Grinch sneaks into Whoville on a state-of-the-art sleigh. His first stop leads him to Cindy Lou Who's home, where, upon being caught in the act of stealing their Christmas tree, he deceitfully claims to be Santa on a mission for repairs and escapes unscathed. Methodically, he proceeds to strip the town of its Christmas joy, cramming everything into a massive sack and whisking it away to the summit of Mount Crumpit, where he plans to dispose of it all.
On Christmas morning, the Whos awaken to the grim reality of their missing treasures, leading to a wave of devastation across the town. Mayor May Who, in his misdirected anger, blames Cindy for giving the Grinch the opportunity to ruin their holiday. However, her father, Lou Lou Who, the postmaster, stands up for her, reminding the townspeople that the spirit of Christmas transcends gifts and decorations—it's fundamentally about love and kindness. Inspired by his heartfelt message, the Whos unite, hold hands, and begin to sing Christmas carols, demonstrating that their spirits remain intact and unbroken.
As the Grinch prepares to hurl the enormous sack off the edge of the mountain, he is struck by the sound of the Whos singing beneath him, which awakens a realization that his malicious scheme has failed. Touched by the true meaning of Christmas, the Grinch feels an unprecedented emotional surge as his heart grows three sizes. In an unexpected turn, as the sleigh, along with Cindy Lou Who — who had climbed aboard — starts to slip towards the cliff's edge, the Grinch leaps into action. He heroically saves both Cindy and the sleigh, ensuring the safe return of all the stolen Christmas goods to the grateful town.
In Whoville, the Grinch humbly apologizes and turns himself in to the authorities, who decide to forgive him, despite May Who's insistence on having him arrested. Tired of Augustus’s self-serving nature, Martha May boldly returns his engagement ring and declares her affection for the Grinch. Ultimately, a transformed Grinch invites the Whos to celebrate Christmas in his cave, where he takes on the role of the festive host, personally carving the Roast Beast for the festive feast, and finally finding his rightful place within the warmth of the Whoville community.
