It is a small seedling tree that was growing in a broken fridge on Earth. It was discovered by WALL•E after he split the fridges door in two with his head-mounted laser. Despite it being a plant, it plays a very large role in the film.
When WALL•E sees the plant, he digs it up and puts it in an old boot he had found earlier. WALL•E takes the plant to the WALL•E transport vehicle where he lives and where he takes all the interesting items he finds.
When WALL•E shows the plant to EVE, her programming identifies the plant and causes her to take the plant and store in a protected space inside her body, she then goes into a stand-by state, showing only a glowing green leaf symbol and waits for her spacecraft to return. WALL•E tries to wake her up but all his attempts fail and he becomes sad as he thinks that he might have lost the one he loves. The ship returns whilst WALL•E is away crushing garbage and he races back to EVE. A robotic arm loads her onto the ship and WALL•E manages to climb onto the side ship before it takes off, but orders Hal, the cockroach, to stay on Earth. There are four other EVE units onboard the probe ship but WALL•E can identify his EVE by the glowing green leaf symbol on her chest.
After an indeterminate amout of time in space, the probe ship arrives at a much larger spacecraft a BNL starliner called the Axiom. On board the Axiom the probe ship unloads the EVE units and WALL•E falls off the ship once gravity is re-engaged in the Axioms docking area. He manages to reach the EVEs shortly before a security robot called GO-4 notices that one of the EVEs had found a plant and summons a cart like transport robot called MVR•A to take the EVE away. GO-4 hops on the transport bot and a robotic arm loads EVE onto it. WALL•E immediately goes after the MVR•A and eventually reaches it.
MVR•A takes GO-4, EVE and WALL•E to a room where they meet an organizing bot called TYP-E. TYP-E allows them to enter an elevator that takes them to the bridge of the Axiom. There they meet the ship's steering wheel and autopilot called Auto. Auto talks with GO-4 and opens a hole in the floor that he uses to reach the captain who is in the lower room. WALL•E falls to the lower room because he was standing on the hole. Auto awakens the captain and the captain's hoverchair takes him to the bridge. He also accidentally pushes WALL•E to the bridge with him.
On the bridge, Auto awakens EVE in order for the captain to witness the plant specimen and then plot a course back to Earth. But after the captain "orders" the ship's computer to open EVE's cavity they see it is empty. The captain is a bit nonplussed and asks Auto if they missed something, but Auto says that EVE's memory is defective and she had not found a plant at all.
EVE and WALL•E are taken to the ships robot repair deck where EVE is examined for repairs and WALL•E is waiting to be cleaned, but WALL•E believes EVE is being hurt and begins to wreak havoc on the deck, eventually leading to EVE deciding to put him into an escape pod and send him back to Earth. EVE and WALL•E are interrupted by GO-4 who enters the room. EVE and WALL•E hide and see GO-4 placing the plant in the escape pod.
GO-4 tries to kill the plant by blowing it up with the escape pod,. WALL•E enters the pod and takes the plant but before he manages to leave the pod GO-4 launches it into space and activates its self-destruct sequence. EVE uses a service airlock to go after the pod and GO-4 leaves the room. WALL•E manages to escape the pod using a fire extinguisher before the pod explodes and saves himself and the plant. He shows the plant to EVE and she takes the plant back. They fly around the Axiom and return aboard and accidentally lock a welder robot outside the starliner.
EVE manages to reach the captain and gives the plant to him. Captain is eager to return to Earth and summons Auto to see the plant, but Auto insists the captain give the plant to him and it is revealed that Auto has a secret, 700 year old Directive A-113 by BnL CEO Shelby Forthright that autopilots should not allow starliners to return to Earth because life was no longer sustainable on the planet. The captain says that life is now clearly sustainable and the existence of the plant proves it, if one plant can survive on Earth, then that certainly proves that Earth is habitable.
Auto is unable to disobey his directive and does not allow the captain to take the Axiom back to Earth. Auto orders GO-4 to take the plant and throw it into the garbage chute. He does so, but the plant is saved since it lands on WALL•E's head as he was climbing up the chute to reach EVE. When WALL•E appears from the chute Auto demand he hand over the plant, WALL•E refuses so Auto shocks him with his taser, WALL•E and the plant fall down the chute. Auto deactivates EVE using her red boot and drops her down the chute as well.Eventually, EVE and WALL•E manage to get the plant to the Axiom's holo-detector and the captain turns Auto off. The Axiom returns to Earth and humans plant the plant into the soil. During the credits of the movie, EVE and WALL•E are observing a large tree and it is revealed that the tree is the same plant that WALL•E had found in the fridge.