Young Carl Fredricksen (Jeremy Leary), a shy boy donning an old pilot's cap and goggles, becomes captivated while watching a film featuring his lifelong idol, Charles Muntz (Christopher Plummer), a legendary explorer. During the screening, the narrator recounts Muntz's daring exploits and remarkable discoveries, only to reveal that scientists have recently discredited his latest find—a large bird skeleton—which they claim is a fabricated hoax. Determined to vindicate himself, Muntz prepares to embark on an expedition aboard his zeppelin, vowing to return with living proof of his discovery.
Following the show, Carl excitedly races down the street with a balloon representing Muntz's zeppelin, The Spirit of Adventure. He passes by an old and dilapidated house, where he is drawn in by someone exclaiming Muntz's iconic phrase: "Adventure is out there!" Curious, Carl ventures inside and encounters a spirited young girl named Ellie, who shares his enthusiasm for exploration and admiration for Muntz. While Carl is taken aback by her boisterous nature, Ellie helps him retrieve his runaway balloon. In a twist of fate, Carl tumbles from a beam and ends up with a broken arm.
That night, Ellie sneaks into Carl's room and reveals her adventure book, expressing her dream of moving to the top of Paradise Falls in South America, complete with a picture that she affectionately “ripped right out of a library book.” She makes Carl promise that they will make that journey together someday. A beautifully crafted montage depicts the blossoming love between Carl and Ellie as they marry and build a life together in the house where they first met. They lead a joyful life, working as a balloon salesman and zookeeper, until they are confronted with the painful reality that Ellie cannot bear children. They resolve to save for their dream trip to Paradise Falls, yet, over the years, unforeseen circumstances chip away at their savings.
One day, Carl, now elderly, realizes that despite their happy life together, they never fulfilled their childhood promise. He plans a surprise picnic for Ellie, complete with tickets to South America, but tragedy strikes—Ellie's health declines, leading to a hospital stay, and ultimately, her passing leaves Carl heartbroken and alone.
Now a reclusive and bitter old man, Carl finds himself reluctant to leave his cherished home as the city rapidly encroaches upon him. Facing pressure to move to a retirement home, he often clashes with the foreman (John Ratzenberger) overseeing the construction. One fateful encounter introduces him to Russell (Jordan Nagai), a gregarious young boy determined to earn his "assisting the elderly" badge. Carl cleverly tricks Russell into searching for a fictitious creature called a "snipe," only for chaos to ensue when a construction worker accidentally damages Carl’s mailbox—an irreplaceable piece of his life with Ellie. This unfortunate event leads Carl to court, compelling him to vacate his beloved home the very next day.
On the morning of his departure, Carl releases millions of helium-filled balloons that lift his house from its foundation, transporting it above the city and into the sky. Free from the city's grip, Carl sets a course for South America. However, things take a sudden turn when Carl discovers Russell clinging to his porch, having followed him in pursuit of the elusive "snipe." Choosing to descend back to safety before an approaching storm, the two find their journey surprisingly tumultuous.
Their descent lands them atop an elevated plateau opposite Paradise Falls, but they become stranded when their helium balloons get snagged. As they attempt to reach the falls before their balloons deflate completely, Russell, on a bathroom break, encounters a colorful giant bird he names Kevin. The duo forges an unexpected bond with Kevin, only to be pursued by a pack of dogs, led by a lighthearted but misguided tracker named Dug (Bob Peterson), who humorously reveals that he is searching for the bird.
As the adventure unfolds, the trio becomes entangled in a chase led by Alpha (Bob Peterson), a doberman, alongside his companions, pursuing Kevin on behalf of their master. Eventually, Carl and Russell's journey leads them to the revelation that their antagonist is none other than an unhinged Charles Muntz, who has hunted Kevin with an obsession fuelled by his need to prove he found the legendary bird.
With thrilling encounters including Muntz's confrontations and the bond strengthening between Carl, Russell, and Kevin, Carl’s resolve strengthens. Through incredible challenges and personal growth, Carl learns the importance of adventure, friendship, and cherishing memories with Ellie. As their journey progresses, Carl comes to terms with letting go of the past, ultimately finding his place amidst new friendships in the heart of a vibrant story that proves that "Adventure is out there!"
In a touching conclusion, Carl not only unites Kevin with her chicks but also becomes a father figure to Russell, symbolically passing down the joys of adventure and discovery, all as he finally sets his old house down where it was meant to be, overlooking Paradise Falls.
