In 1940, Carl Fredricksen is a young 9-year-old boy who idolizes famous explorer Charles Muntz. Muntz has been accused of fabricating the skeleton of a giant exotic bird he says he discovered at Paradise Falls, and vows to return there to catch one alive. One day, Carl befriends a girl named Ellie, who is also a fan of Muntz. 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 eventually get married and live together in the restored house. Carl sells toy balloons from a cart at a zoo Ellie opens. After suffering a miscarriage and being told they cannot have a child, the two decide to realize their dream of visiting Paradise Falls. They try to save for the trip, but repeatedly end up spending the money on more pressing needs. Finally, an elderly Carl arranges for the trip, but Ellie suddenly becomes ill and dies.
Years later, Carl still lives in the house, stubbornly holding out as the surrounding neighborhood is torn down for new construction, but when he accidentally injures a construction worker over damage to his mailbox, a court orders him to move to a retirement home. However, Carl comes up with a scheme to keep his promise to Ellie, and turns his house into a makeshift airship, using thousands of helium balloons. Russell, a young Wilderness Explorer, becomes an accidental stowaway in his effort to earn his final merit badge for assisting the elderly. After surviving a thunderstorm, the flying house lands on a tepui opposite Paradise Falls. Carl and Russell harness themselves to the still-buoyant house and begin to walk it across the mesa, hoping to reach the falls before the balloons deflate. Russell encounters a tall, colorful flightless bird, whom he names "Kevin". They then meet a Golden Retriever named Dug, who wears a special collar that allows him to speak, and who vows to take the bird to his master.
The group is set upon by a pack of aggressive dogs led by Alpha, a doberman (who is also seeking the bird), and are taken to their master, who turns out to be an elderly Charles Muntz. Muntz invites Carl and Russell aboard his dirigible, where he explains that he has spent years since his disgrace searching for the giant bird. When Russell notes the bird's similarity to Kevin, Muntz becomes hostile, believing they have been attempting to steal the bird. The pair flees with Kevin and Dug, but Muntz catches up with them, captures Kevin and starts a fire beneath Carl's house, forcing him to choose between saving it or Kevin. Carl rushes to put out the fire, allowing Muntz to take the bird. He and Russell eventually reach the falls, though Russell is disappointed in Carl over his decision to abandon Kevin.
Settling into his home, Carl looks through Ellie's childhood scrapbook, and is surprised to find that she has filled in the blank pages with photos of their marriage, along with a note thanking him for the "adventure" and encouraging him to go have a new one. Reinvigorated, he goes to find Russell, only to see him sailing off with some balloons to save Kevin on his own. Carl empties the house of furniture and possessions, lightening it, and pursues him. Russell is captured by Muntz, but Carl manages to board the dirigible in flight and free both him and Kevin. Dug accidentally defeats Alpha and becomes the dogs' new leader. Muntz pursues them around the airship, finally cornering Dug, Kevin, and Russell inside Carl's tethered house. Carl lures Kevin back onto the airship with Dug and Russell clinging to her back, but when Muntz leaps after them, he snags his foot on some balloon lines and falls to his death. The house then descends out of sight through the clouds.
Carl and Russell reunite Kevin with her chicks, then fly the dirigible back to the city. Carl presents Russell with his final badge: a grape soda cap that Ellie gave to Carl when they first met and made their promise. The two and Dug then enjoy some ice cream together. Meanwhile, Carl's house has landed on the cliff beside Paradise Falls, fulfilling his promise to Ellie.
Output: Why did Muntz want to return to Paradise Falls?