In the enchanting Kingdom of Cloister, a young farm boy named Jack is captivated by the tale of Erik, an ancient king who once vanquished an invading army of gigantic creatures using a magical crown. This crown, forged from the heart of a slain giant, allowed Erik to command the giants, who had breached the Earth through a beanstalk sprouted from a mystical seed. The giants descended to plunder, but with the cunning use of his crown, Erik succeeded in sending them back to their skyward realm. Years pass, and when Erik dies, he is interred alongside the crown and the remaining magic beans—the only ties connecting Earth to the giant kingdom. Around the same time, Princess Isabelle becomes enthralled by the same legendary stories.
A decade later, Jack ventures into town to sell his horse to help sustain his uncle's farm. There, he encounters Isabelle and instantly develops feelings for her, bravely stepping in to defend her from some unruly ruffians. Meanwhile, Lord Roderick, who is betrothed to Isabelle, discovers that a monk has pilfered his belongings, including Erik's magical crown, though the precious beans have mysteriously vanished. In response, he imposes a city-wide lockdown.
The monk, desperate to escape, offers Jack some of the stolen magic beans in exchange for his horse. He urges Jack to deliver the beans to the abbey for their value. Back at the palace, Isabelle argues with her father, King Brahmwell, yearning for freedom and exploration while he insists she remain and marry Roderick. At the same time, Jack's uncle chastises him for his foolishness in accepting beans as payment and carelessly discards them. However, one of the beans finds its way into the basement.
Longing for adventure, Isabelle sneaks away from the castle, seeking refuge from the rain in Jack's home. During the storm, one of the magic beans germinates and rapidly grows into a colossal beanstalk, lifting Jack's house—and Isabelle—into the sky. In a noble attempt to save her, Jack tumbles down, clutching only Isabelle's bracelet.
The following day, the King's party discovers Jack unconscious, clutching the bracelet that belonged to Isabelle. When he recounts his tale, Roderick realizes that the beanstalk's growth was caused by the bean's contact with water. Determined to rescue the princess, Jack, Roderick, and Roderick's aide Wicke pledge to join the king's knights, led by Elmont and his trusted companion, Crawe. However, they soon find themselves in peril when Roderick and Wicke sabotage the group's safety.
Arriving in the giants' domain, they split into two factions: one with Jack, Elmont, and Crawe, and the other led by Roderick. During their quest, Jack's group is taken captive by a giant, whereas Roderick's team confronts two more giants, one of whom devours Wicke. In a desperate gamble, Roderick dons the magic crown, asserting his dominance over the giants.
In a climactic showdown within the stronghold of Fallon, the two-headed leader of the giants, Jack uncovers Isabelle and Elmont held prisoner. As chaos ensues, Roderick uses the crown to enslave the giants, plotting an assault on Cloister at dawn while he plans to feed Isabelle and Elmont to them. Unfazed, Jack valiantly rescues them just moments before they become a meal.
With quick thinking, they flee towards the beanstalk while Jack bravely dispatches the giant blocking their escape. King Brahmwell, witnessing the giant's fallen form, orders the beanstalk to be cut down to prevent an invasion. Jack and Isabelle descend safely, while Elmont confronts Roderick, ultimately defeating him, though Fallon snatches the crown during the conflict, forcing Elmont to retreat.
Returning to Cloister, Jack is alarmed to learn that Roderick’s beans are still in use, enabling the giants to construct more beanstalks for an attack. The giants storm the castle, where Elmont ignites the moat, setting it ablaze. As the battle rages on, Jack throws the last magical bean down Fallon's throat, creating a beanstalk that disrupts the giant's body.
With the giants vanquished, Jack claims the crown and restores peace. Eventually, he and Isabelle marry and share the fabled stories of the giants with their children. Over time, the magic crown is transformed into St Edward's Crown and is secured in the Tower of London, forever marking the legacy of their epic journey.
