WALL-E is one of several thousand WALL-E model robots tasked with cleaning up the filthy abandoned Earth after humanity's departure. Somehow, WALL-E remained active after his fellows were deactivated when Buy N' Large decided the Earth clearly could not recover and support life again, and was left following his programming alone.
Some 700 years later, WALL-E was still doing his assigned duty, though had now developed sentience. He had a friend of sorts in the form of a cockroach named Hal, a love for musicals (specifically Hello Dolly, which he watched religiously), a collection of human trinkets and knick-knacks, even if he wasn't sure what they did, and a feeling that something was missing in his life. He was now performing maintenance on himself by removing parts from his non-functional fellows dotted around the landscape, both hording spare parts in the truck he lived in and doing repair work out in the open if it was absolute essential, such as when he needed new treads. On the same day he came across a young sapling, he encountered a sleek, more advanced robot named EVE. WALL-E immediately fell in love with her and was able to lead her back to his trailer, where he proudly showed off his collection, though was distraught when she sealed the plant into her chest and entered stasis. Not to be deterred in winning her heart, WALL-E took the silent EVE out on a number of 'dates' and even followed her back to her ship of origin, the Axiom, clinging to the exterior of the rocket taking her back.
In his journey through the Axiom in search of his beloved, WALL-E unintentionally made differences to a lot of lives, both human and robotic. He unwittingly showed two humans, John and Mary, the world around them which they'd never noticed before, but also caused no end of trouble for BURN-E, a maintenance robot, and M-O, a tenacious neat freak robot who followed WALL-E all over the ship, desperate to clean him. WALL-E, unfortunately, was electrocuted, caught fire, and badly damaged by AUTO, the ship's autopilot, when he wouldn't surrender the sapling so the Axiom could return to Earth. Tossed into the ship's garbage disposal with EVE, M-O and the plant, the dying WALL-E refused to let EVE scour the junkpiles for spare circuit boards, instead convincing her to place the sapling into a storage bay, automatically returning it to Earth, where he could be repaired with the spare parts in his collection. AUTO tried closing the storage bay before the plant could be placed inside, but WALL-E was able to keep it open long enough for the plant to be placed inside, though he was almost crushed and fatally wounded.
Upon the Axiom's return to Earth, EVE returned the ruined WALL-E to his trailer and frantically set about repairing him. But when the repairs were complete and he was fully recharged, it seemed as though all his sentience and personality had been lost. He began crushing elements of his collection into cubes, flattened Hal without thinking, and his recordings of Hello Dolly had been wiped. Distraught, EVE held his hand for a moment, which he had long wanted, and gave him a electric kiss before turning to leave. She was surprised to find that he wouldn't let go. His hand tightened around hers and his eyes switched back to their usual position as he recognised his beloved, and was overjoyed to find that they were finally holding hands. As humanity rebuilt their lives on Earth with the help of their robots, WALL-E played his part in the renewal of life on Earth, and was last seen in a lush field with EVE, gazing up at a tree which had grown from the sapling he had found all those years ago.