He is a 9-year-old American boy who has an argument with his mother after she sets him chores to complete over the summer when he wants it to be fun. After Milo breaks his mother's "No broccoli, no TV" rule by feeding it to the cat and watching TV when he was supposed to eat it, he gets sent to bed early.
But he instead bounces on the bed and, exasperated, wish his life would be easier without her, thus making her very sad. His wish comes true as the Martians come and steal his mother so she can raise their young. In order to rescue his mother, he boards the Martians' spaceship. He eventually meets up with Gribble, who himself lost his mother to the Martians.
After Gribble's reluctance, they eventually decide to rescue Milo's mother before sunrise. They team up with Ki, and together are able to stop the rule of the Supervisor over the Martians and save his mother. Milo and his mother return home, where he now completes his chores much to the surprise of his father.