In 1939, nine-year-old Carl Fredricksen idolizes famous explorer Charles F. Muntz. When Muntz is accused of fabricating the skeleton of a giant exotic bird he says he discovered at Paradise Falls, he vows not to return until he captures one alive. One day, Carl befriends a girl named Ellie, also a Muntz fan. She confides to Carl her desire to move her “clubhouse”—an abandoned house in the neighborhood—to a cliff overlooking Paradise Falls.
Carl and Ellie grow up, get married and live in the restored house. Carl sells toy balloons at the zoo where Ellie works. After she suffers a miscarriage and they are told they cannot have a child, the two decide to realize their dream of visiting Paradise Falls. They save for the trip, but repeatedly have to spend the money on more pressing needs. Finally, the now elderly Carl arranges for the trip, but Ellie suddenly becomes ill and dies.
Years later, Carl still lives in the house, stubbornly holding out while the neighborhood homes are torn down and replaced by skyscrapers. When he accidentally injures a construction worker, the court deems him a public menace, ordering him to move to a retirement home. However, Carl comes up with a scheme to keep his promise to Ellie by turning his house into a makeshift airship, using thousands of helium balloons. Russell, a young Wilderness Explorer who visited Carl in his effort to earn his final merit badge, for assisting the elderly, becomes an accidental stowaway.