In the Kingdom of Cloister, in a time long past, monks craft magic beans that grow into a large beanstalk. They reach the realm of Gantua, home to a race of giants. The giants kill the monks and attack the Earth below. Through the use of a magical crown, King Erik manages to control the giants and force them to return, and the beanstalk is cut down. When Erik dies, his crown and the remaining beans are buried with him to keep them safe.

Years later, Jack, an orphaned, 18-year-old peasant, is sent by his uncle to sell his horse, Anser, and his cart at market. There, he runs into a disguised Princess Isabelle, whom he defends from an attack. The royal guard takes Isabel back to the castle. Afterwards, Jack realizes that his cart was stolen.

Meanwhile, Lord Roderick, second in command to King Brahmwell, has collected the beans and crown. A monk steals the beans, runs into Jack while trying to escape, and exchanges the beans for Anser. Warning Jack not to get the beans wet, the monk flees, pursued by the guards.

Isabel later sneaks out of the castle again, but is caught in a storm before coming upon Jack’s door. While they talk, the rain leaks into the house and wets a bean. It grows into a massive beanstalk that traps Isabel in the house as it is lifted into the sky. Jack attempts to free her, but is knocked away. The next morning, Brahmwell sends Roderick and other knights up the beanstalk to retrieve Isabel; Jack volunteers to join. During the climb, Roderick takes advantage of an accident to secretly cause most of the knight’s deaths. After reaching Gantua, Roderick corners Jack and takes the remaining beans from him, though Jack secretly keeps one.

The group splits up. Knights Elmont and Crawe are knocked out and taken captive by a giant. Roderick pushes knight Bald down a cliff. When another giant appears and eats servant Wicke, Roderick reveals the crown and forces the giants to obey him. Elmont and Crawe are taken to the leader of the giants, Fallon, and are reunited with Isabel. Fallon eats Crawe before Roderick arrives wearing the crown and takes command. He proclaims himself king of the giants, orders that Isabel and Elmont be eaten, and has the giants prepare to descend the beanstalk and conquer the world for him.

Jack eventually kills the chef who was to cook Isabel and Elmont. The three flee towards the beanstalk, but find their way blocked by another giant. Jack and Elmont trick him into falling off the edge. When the giant hits the ground, the body is seen by Brahmwell. Fearing a giant invasion, he orders the beanstalk cut down.

Jack and Isabel climb down the beanstalk while Elmont stays behind to fight Roderick, killing him. Fallon then takes the crown, claiming the title of king. Meanwhile, the beanstalk falls away and Elmont rides it to the ground. Isabel is reunited with Brahmwell, who rewards Jack for her rescue. The group departs and Jack remains behind.

Fallon finds the remaining beans and uses them to grown more beanstalks, which the giants ride to the ground. Reuniting with Anser, Jack sees the new beanstalks approaching and decides to warn Isabel. After they reach the safety of the city walls, the drawbridge is raised and oil is poured into the moat to light it on fire. Fallon, initially seeming to drown in the fiery moat, finds an underwater passageway into the catacombs under the castle, breaking into the throne room and capturing Jack and Isabel in the process. Jack throws the final bean down Fallon’s throat. The bean grows into another beanstalk, ripping Fallon apart. Jack takes the crown and uses it to stop the giants’ attack.

The giants are sent back to Gantua and forced to cut down their beanstalks. Brahmwell allows Jack and Isabelle to marry, who eventually have twin children. Their story becomes a popular fairy tale and the crown is secretly crafted into St Edward’s Crown. In the present day Tower of London, a boy, Roderick’s descendant, gazes at the crown. Meanwhile, Gantua still exists above London.