Carl Fredrickson, as a young boy, idolizes famed explorer Charles F. Muntz, who vows to venture into Paradise Falls and capture a rare bird species after being accused of being a fraud. Carl befriends a girl named Ellie and marries her years later, and promises for the two of them to journey to Paradise Falls. However, Ellie passes away years later.
Carl, now a recluse, lives in a construction site. After injuring a construction worker, Carl is forced to have to move into a retirement home. However, Carl, using thousands of helium balloons, lifts his house into the air and sets sail to Paradise Falls, but discovers that a young wilderness explorer named Russell has stowed away. The two eventually land in South America, and Carl resolves to drag the floating house to Paradise Falls. During their journey, the two meet a bird whom Russell dubs Kevin, and a dog named Dug, who can talk due to an enhanced collar.
Dug's pack, led by a dog named Alpha, captures them and escorts them to a gorge to meet their master, who Carl discovers to be an elderly Charles Muntz. Muntz welcomes them initially, but turns hostile upon learning of their knowledge fo Kevin, who Muntz has spent years searching for. Carl, Russell, and Kevin escape, aided by Dug, but Kevin is injured in the process. Carl resolves to return Kevin to the canyon where she lives in.
Kevin is captured by Muntz, who had followed them using Dug's collar. As a distraction, Muntz sets Carl's house aflame. In his rage, Carl drives Dug away before finally arriving at Paradise Falls. Carl's settles into the house and reads Ellie's adventure book, where he notices a message on the last page, encouraging him to go on a new adventure. Carl sees Russell set off after Muntz, and follows him by lightening the house by removing several items from it. Carl and Dug set off after Muntz's dirigible, where they rescue Kevin and Russell. Muntz pursues them, damaging the house with his shotgun and destroying its ability to stay airloft. Ultimately, Muntz plummets to his death as he tries to capture Kevin. The house, meanwhile, floats out of view.
After Kevin is returned to her chicks, Carl and Russell fly the dirigible back to civilization. In the city, Carl attends Russell's senior explorer ceremony, awarding him with a grape soda badge given to him by Ellie. Meanwhile, the house lands back on Paradise Falls.
- Edward Asner as Carl Fredrickson.
- Christopher Plummer as Charles F. Muntz.
- Jordan Nagai as Russell.
- Bob Peterson as Dug/Alpha.
- Delroy Lindo as Beta.
- Jerome Ranft as Gamma.
- John Ratzenberger as Construction Foreman Tom.
- Elie Docter as Ellie.
- Mickie McGowan as Police Officer Edith.
- Danny Mann as Construction Worker Steve.