M-O remembered, with the absolute clarity of computer recall, his life onboard the Axiom. He remembered his immaculate and tiny station with the other cleaner bots, the gleaming floors, the pristine walls. Only the EVE probes brought foreign contaminants onboard; only they had the opportunity to come in contact with such things. So, once a year, M-O woke, zoomed out and along his light path, and cleaned the soil from the probes. The rest of the year he made a daily round of the bay, following his light path, checking for nonexistent contaminants and returning to his station for hibernation. For hundreds of years, this was his routine.
And then, HE came. He, with his filthy treads leaving horrible tracks all over the ship and on M-O's face screen, and his waving at robots-- at robots!! Robots didn't have senses of humor, or social impulses, or self-set directives-- and they certainly did not leave foreign contaminant tracks all over the ship on M-O's watch. The newcomer was a problem, clear and simple, a reject-bot who had escaped from the repair ward, most likely. And he left a trail of earth all over M-O's perfectly clean world.
M-O also remembered with photographic detail everything that had followed. He had wound up in the absolute worst place he could imagine, the one place he never should have been-- the garbage disposal bay. His programmer would have had fits had he known the tidy little cleaner bot would one day be splattered with grime from garbage heaps. And he had stood in the light of the rusty, filthy Wall-As, finally cleaning the erratic newcomer to his heart's content.
And that was the irony, of course. That the foreign reject-bot was working correctly, and that M-O had not been, not until he had left his light path to choose his own course…and that, after leading him a merry, dirty chase, the foreign bot liked him and even learned his name and shook his roller. A lot of other bots learned how to work correctly that day, too, and M-O learned all of their names, and didn't feel a need to scan them for foreign contaminants.
So, once, he was a spotless little cleaner bot with a perfectly-sized station onboard a magnificently appointed ship, and everything was clean and gleaming. He looked down at himself now, at a bit of earth that had gathered above his roller ball, and he buffed it away and hummed cheerily to himself, a song about Sunday clothes, something you wore to make yourself feel good. So what if he was no longer spotless? If he no longer had a gleaming station to hibernate in? He snuggled himself deeper into the cubbyhole on the rotating rack of Wall-E and EVE's truck. Hal scrambled over his face screen on his way to the floor, tickling M-O's brush as he passed, and M-O giggled. His surroundings were filthy, true. He shared a home with an EVE probe, a garbage-compacting reject-bot and a cockroach, yes. He slept on a rack with hundreds of items pulled out of the detritus of an entire planet. There were certainly no gleaming decks here.
But neither were there any light paths.
M-O was free to choose his own course. And it always led back here, to his friends.