Stanley is a cheerful Troll with the ability to grow sentient flowers using his magical green thumb. He lives in the Kingdom of Trolls, ruled by the cruel Gnorga, a tyrant who despises blooms and can turn almost anything to stone with her purple thumb. Because his gift is a secret among the trolls, Stanley keeps it hidden to avoid trouble in his community.
One day, a mishap exposes Stanley's living bouquet to the trolls, and he is dragged before Gnorga for punishment. She wants him turned to stone, but her husband, Llort, persuades her to banish him instead and send him far away to New York City.
Stanley arrives in New York, frightened by his new surroundings, and ends up living in a cave under a Central Park bridge.
Meanwhile, a boy named Gus and his baby sister Rosie feel neglected by their busy parents and slip away to explore Central Park. When they meet Stanley, he demonstrates his floral magic.
Gus is suspicious of the stranger and tries to pull Rosie back home, sparking a fight that sends Rosie crying.
Back in the Troll Kingdom, Gnorga watches the scene and grows furious that Stanley is happy in exile. She crafts a plan and casts a spell on Gus that makes him cry uncontrollably, flooding Stanley's cave with tears.
Using his green thumb, Stanley inflates Gus and Rosie's toy boat into a living craft, and the trio climbs aboard to escape.
Longing to reclaim control, Gnorga and Llort ride a tornado to Central Park, leaving chaos behind.
When Gus decides to confront the threat, he asks Stanley for help, but a frightened Stanley refuses. Frustrated, Gus storms off, determined to face Gnorga with the flowers following behind.
At an abandoned building where Rosie is held, Gus and the flowers free Rosie and battle Gnorga and Llort, the latter of whom fights alongside his ally. Gnorga transforms Gus into a troll with stone powers.
During the battle, Rosie falls from a building but is saved by Stanley, who uses his powers to give Gus and Rosie's boat the ability to fly.
Stanley then faces Gnorga in a thumb-wrestling match and gradually turns her into a flowering bush, before he flees with Gus, Rosie, and the flowers; before fully succumbing to her curse, Gnorga manipulates Gus into turning Stanley to stone.
After Stanley is turned to stone, Gus and Rosie return home, with the former regaining his human form. The next morning, Gus, Rosie, and their parents visit Central Park, where Gus and Rosie place the petrified Stanley on a makeshift pedestal. Using Stanley's green-thumb trick, Gus revives Stanley, who uses his magic to restore Central Park and cover New York City in flowers.
