In the Kingdom of Cloister, Jack, a young farm boy, is fascinated by the legend of Erik, an ancient king who defeated an army of invading giants from a realm in the sky by controlling them with a magical crown. At the same time, Princess Isabelle becomes fascinated with the same legend.
Ten years later, Jack goes into town to sell his horse to support his uncle’s farm. There, Jack spots Isabelle and becomes enamored with her after defending her honor from a group of hooligans. Meanwhile, Lord Roderick returns to his study, only to find that a monk has robbed him. The monk offers Jack some magic beans he stole from Roderick as collateral for Jack’s horse. Back at the castle, Isabelle quarrels with her father King Brahmwell as she wants to explore the kingdom, but he wants her to stay and marry Roderick. Likewise, Jack’s uncle scolds him for being foolish before throwing the beans on the floor and leaving the house.
Determined to be free, Isabelle sneaks out of the castle and seeks shelter from the rain in Jack’s house. As it rains, one of the beans takes root and grows into a massive beanstalk that carries the house and Isabelle into the sky as Jack falls to the ground.
Jack, Roderick, and Roderick’s attendant Wicke volunteer to join the king’s knights, led by Elmont and his second in-command, Crawe, and climb the beanstalk in search of Isabelle. As they climb, Roderick and Wicke cut the safety rope, intentionally killing some of the knights. At the top, they discover the giants’ realm and decide to split into two groups: one with Jack, Elmont, and Crawe, and the other including Roderick and Wicke, but not before Roderick forcibly takes the remaining beans from Jack (although Jack manages to save one for himself).
Jack’s group is trapped by a giant, who takes everyone prisoner except Jack. Meanwhile, Roderick’s group encounters two other giants; one eats Wicke, but before they can do the same to Roderick, Roderick dons the magic crown.
Jack follows the giant to their stronghold, where the two-headed giant leader, Fallon, has killed Crawe. There, Jack finds the imprisoned Isabelle and Elmont. As the giants prepare to kill their remaining prisoners, Roderick walks in and enslaves the giants with the crown. He tells the giants they will attack Cloister at dawn and gives them permission to eat Isabelle and Elmont. Jack rescues Isabelle and Elmont as one of the giants prepares to cook Elmont as a pig-in-a-blanket. The trio makes for the beanstalk, where Jack causes the giant guarding the beanstalk to fall off the realm’s edge. Seeing the giant’s body, Brahmwell orders the beanstalk cut down to avoid an invasion by the giants.