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With the love of his life gone, widower Carl (Ed Asner) might as well be dead. His grief has soured into guilt, which he walls up in a castle of cantankerousness. His day is a dull routine of dressing, hobbling with his cane to sit on the front porch and keeping his home just as it was when Ellie was there. It's really a mausoleum, and he is both caretaker and corpse. We never heard Carl say a word to Ellie while she was alive, but now he talks nonstop to his absent darling. She'd understand his bitterness; she might even forgive it.
Since this character study is also an action-adventure film, Carl has to go somewhere — Paradise Falls, obviously. But he doesn't have to leave his home. Threatened with eviction to an old folks' home, he attaches 20,000 helium-filled balloons to his house, and off it floats toward South America. But there's a stowaway on board: Russell (Jordan Nagai), a plump, determined kid who has been pestering Carl to let him "assist the elderly," the one good deed he needs to become a full Wilderness Explorer. The old man isn't pleased, but he's not stopping now.
As Docter notes, Up is driven by the idea of escape — the notion, familiar to dreamers of any age, that "you could just float away and take what you want with you." What Carl wants to take is the house where he spent a happy half-century with Ellie and where, in a sense, she still lives. Like a snail or, more likely, Atlas, Carl carries his house and the world's burden on his back; his wish for escape is also a sacred responsibility, to take Ellie to Paradise Falls.