Plot

Young Carl Fredricksen (Ed Asner) is a quiet bespectacled boy wearing an old pilot's cap and goggles who idolizes renowned explorer Charles F. Muntz (Christopher Plummer). He is saddened to learn, however, that Muntz has been accused of fabricating the skeleton of a giant bird he had claimed to have discovered in Paradise Falls, Venezuela, South America, and was fired. Intent on proving them wrong, Muntz is seen boarding his zeppelin called the Spirit of Adventure, with his team of dogs and promises to return once he has brought back living proof of his find. The zeppelin was designed by Muntz himself, and has all the comforts of modern life, including a doggy bath and a mechanical dog walker.

One day, Carl befriends an energetic and somewhat eccentric tomboy named Ellie, who is also a Muntz fan. Ellie invites Carl to join her club, and to commemorate the occasion she gives him a grape soda bottle cap as a badge. Ellie is the outspoken, adventurous type, who pushes Carl out of his comfort zone to try new adventures. She confides to Carl her desire to move her "clubhouse", an abandoned house in the neighborhood, to a cliff overlooking Paradise Falls, making him promise to help her.

Carl and Ellie eventually get married and grow old together in the restored house, working as a toy balloon vendor and a zookeeper, respectively. Carl and Ellie make a beautiful life together, spending time with each other and enjoying nature. Ellie suffers a miscarriage, and it is implied that she is infertile. When they are told by a doctor that they are unable to have children, they save for a trip to Paradise Falls but repeatedly end up spending the money on more pressing needs like a new car tire, Carl's accident (which caused a leg fracture), house repairs when a tree fell on it during a storm and so on. Many years later, a now elderly Carl decides to surprise Ellie with tickets to South America, but she falls ill and is hospitalized, giving Carl her scrapbook before dying soon after.

Now in his late 70s, Carl holds out while the neighborhood around him is redeveloped. He refuses to sell. He ends up injuring a construction worker over damage done to his mailbox. He is evicted from the house by court order due to being deemed a "public menace" and is ordered to move to a retirement home. However, Carl comes up with a scheme to keep his promise to Ellie. Workers from Shady Oaks retirement home arrive to pick him up the following morning but are shocked to find Carl releasing millions of helium balloons into the air which detach his house from its foundation, lifting it over the city and into the sky.

He turns his house into a makeshift airship, using thousands of helium balloons to lift it off its foundation. He uses curtains as sails and the wind pane as a rudder to control direction and speed. Carl sets a course for South America. A 13 year young member of the "Wilderness Explorers", tribe 54, Sweat Lodge 12 (a youth organization) named Russell (Jordan Nagai) becomes an accidental passenger, having pestered Carl earlier in an attempt to earn his final merit badge, "Assisting the Elderly". Russell was missing this last badge to become a senior wilderness explorer. Carl has asked Russell to find a snipe that had been bothering him, and Russell had come over to provide a progress report when Carl unfurled the balloons. Russell was stuck on the front porch of the house. Carl lets him in and decides to descend to return Russell home before a severe storm hits.

After surviving a thunderstorm, the house lands near a large ravine facing Paradise Falls. Carl releases some balloons to descend but they hit ground early and are knocked out of the house. They manage to hold onto it using a hose attached to the porch while the fog lifts to reveal that they are standing on a high plateau opposite Paradise Falls. Unable to climb back into the house, Carl and Russell harness themselves to the still-buoyant house and begin to walk it around the ravine, hoping to reach the falls before the balloons deflate.

Meanwhile, a chase is progressing in the jungle. Three dogs with red lights on their collars are in hot pursuit of what appears to be a giant bird, but they lose the trail when their sensitive ears pick up the fine tuning of Carl's hearing aid.

Carl and Russell later befriend a tall, colorful flightless bird (whom Russell names "Kevin") trying to reach her chicks, and then a dog named Dug (Bob Peterson), whose collar has a device that translates his thoughts into human speech. After failed attempts to evade the animals, Carl reluctantly allows both to join the party. Kevin loudly calls out and is answered by smaller calls. Dug says that Kevin is calling to her babies, and Russell realizes that Kevin is a girl.

Meanwhile, the three dogs seen chasing the bird earlier have picked up the scent of Carl and Russell, who they nickname the mailman. The leader Alpha (Bob Peterson), a Doberman Pinscher, tells Beta (Delroy Lindo), a rottweiler, and Gamma (Jerome Ranft), a bulldog, that they must be vigilant and continue their search. Alpha had sent Dug on a fools errand to keep him occupied, but when he turns on the collar communication device, Alpha realizes that Dug has found the bird and is with Carl and Russell. Alpha turns on the tracking beacon on Dug's collar to find his position. Carl and Russell encounter the pack of dogs led by Alpha, but Kevin has already run off. They are taken to Dug's master, who turns out to be elderly 92 years old Charles Muntz.

Muntz invites Carl and Russell aboard his dirigible, where he explains that he has spent the years since his disgrace searching Paradise Falls for the giant bird. The zeppelin is anchored inside a large and tall cave, where Mintz lives with his dogs. The dogs serve Muntz and even do his cooking for him.

Carl's initial excitement over meeting his hero fades as he becomes aware that Muntz's obsession has driven him insane. After Muntz shows them the bird skeleton, Russell notes its resemblance to Kevin. Muntz says that he has tried to smoke the bird out of its habitat, but sometimes years go by between sightings. The bird's habitat is a labyrinth, which has no way out and Muntz has already lost many dogs in the process. When Russell innocently reveals his friendship with Kevin (indicating that he attracted the bird with chocolate), Muntz becomes disturbingly hostile.

Muntz reveals that he has murdered other travelers he suspected of also seeking the bird. Muntz reveals a table of head mannequins wearing various headgear and grimly knocks each one off with his cane as he describes the stories their wearers told him (a surveyor making a map, a botanist making a catalog of the plants); claiming that each one was actually after his bird. This is when Kevin enters the cave and gives out a call, distracting Muntz. This gives an opportunity to Carl and Russel to run out of the Zeppelin. This prompts Carl, Russell, Kevin and Dug to flee, chased by Muntz's dogs. Riding on Kevin's back and assisted by Dug, who calls Carl his new master, they barely escape capture by Muntz's dogs, though Kevin is injured in the process.

Hearing Kevin call out to her chicks, Carl agrees to take her home. Just before Kevin can re-enter her labyrinth home, a net flies out and captures her. Muntz and his dogs have arrived in the zeppelin, led to the spot by a tracking device on Dug's collar. Muntz eventually catches up with them and starts a fire beneath Carl's house, forcing Carl to choose between saving his home or Kevin. Carl rushes to put out the fire, allowing Muntz to take the bird. Carl and Russell eventually reach the falls, but Russell is angry with Carl.

Settling into his home, Carl discovers photos of their married life in Ellie's childhood scrapbook and a final note from his wife thanking him for the "adventure" and encouraging him to go on a new one. Reinvigorated, he goes to find Russell, only to see him sailing off on some balloons to rescue Kevin. Carl empties the house of his furniture and possessions and pursues him.

Russell is captured by Muntz, but Carl boards the dirigible in flight and frees both Russell and Kevin. Muntz pursues them around the airship, finally cornering Dug, Kevin, and Russell inside Carl's tethered house. Carl lures Kevin out through a window and back onto the airship with Dug and Russell clinging to her back, just as Muntz is about to close in, Muntz leaps after them, only to snag his foot on some balloon lines and fall to his death. Snapped from its tether, the house descends out of sight through the clouds, which Carl accepts as being for the best.

Carl, Russell and Dug reunite Kevin with her chicks, then fly the dirigible back to the city. When Russell's father misses his son's Senior Explorer ceremony, Carl proudly presents Russell with his final badge for assisting the elderly, as well as a personal addition: the grape soda cap that Ellie gave to Carl when they first met (which he dubs the "Ellie Badge").

Meanwhile, Carl's house is shown to have landed on the cliff beside Paradise Falls, as promised to Ellie. A series of photographs shows Carl enjoying his latest adventure: living an active life as a surrogate grandfather to Russell.