M-O is obsessed of keeping things exempt of "Foreign Contaminant", and can do anything to complete this task (an example is when WALL-E teasingly wipes the dirt off one of his treads and covers M-O's face with it, he screams and franctically tries to clean his own face!) He travels around on a single roller, equipped with cleaning tools.
M-O first appears after EVE and WALL•E arrive at the Axiom. He then scans the robots for any "foreign contaminants" and is shocked when his detector yields WALL•E 100% "foreign contaminant." He then attempts to clean WALL•E, but WALL•E does not understand what M-O is saying. However, WALL•E then leaves to go after EVE, leaving dirty tracks behind. M-O encounters those tracks, beginning a grueling process of cleaning them up. Following the tracks, M-O eventually comes across a garbage chute, which leads him into a Garbage Disposal chamber. Searching across the chamber, he detects a "foreign contaminant," finally leading him to WALL•E, whom EVE is trying to free from a large trash cube he is stuck in. M-O rushes toward the two robots and manages to wedge himself between the closing inner airlock doors just as the outer airlock doors open, sending garbage out into space. EVE, holding a damaged WALL•E (tasered by AUTO), manages to grab hold onto M-O, and the three are saved when WALL•As, who take notice, shut the outer airlock doors. M-O then fully cleans WALL•E, and the two introduce each other. After WALL•E shows EVE his damaged motherboard and tells her that his replacement parts are back down at Earth, M-O accompanies the two as the three leave the Garbage Disposal chamber and, joined by reject bots (that WALL•E has encountered earlier in the ship's Repair Ward), make their way to the holo-detector on the Lido Deck, where when the plant is placed, the Axiom will jump to Earth. However, WALL•E accidentally drops the plant when he sideswipes the holo-detector, and the plant is thought to be lost until M-O, when searching across the Lido Deck, detects another "foreign contaminant," which happens to be the plant. Picking up the plant, M-O has the reject bots and humans form a line to pass it all the way up to EVE. He and the rejects lower their heads as they mourn the "death" of WALL•E, who has gotten crushed and severely damaged in a valiant effort to keep the holo-detector open (AUTO, unable to disobey his directive, had closed the holo-detector on WALL•E before being finally deactivated by the ship's captain).
After the Axiom arrive back on Earth, M-O, with the reject bots, follow EVE, who rushes WALL•E back to his truck. They stop as they see a roof of the truck explode (EVE has blasted a hole in the roof to give WALL•E sunlight to recharge). Arriving at the scene where EVE and WALL•E are holding hands and staring into each other's eyes, M-O and the rejects celebrate WALL•E's revival (EVE has repaired WALL•E and restored his memory with a kiss). M-O then ushers the other robots into giving the two some privacy, allowing both robots to share a tender moment.