The Mediocre Ability Users are a trio of criminals who inexplicably have obtained supernatural abilities. However, as their title would imply, they aren't very powerful.
The group meet up and discuss their plan to pull off a crime in a public restroom, though they note that they spoke with one other guy online who should be there. They soon run into Saitama and mistake him for their ally. However, when he introduces himself as a hero, they attack him. Their attacks misfire completely and they end up getting saved from themselves by Saitama, and start contemplating about how weak their abilities are compared to the likes of Tatsumaki, Fubuki, and Volten, who use their respective abilities to earn a living. They have a change of heart and decide to work together to make it big. However, the group is then attacked by Dirt Earthworm, who tries to crush them with a wall of earth. They are saved by Saitama again, who sends the monster flying. Genos arrives at the scene after following the evacuation warning, only to find Saitama there alone with The Mediocre Ability Users, who he thinks are troubling his master. They quickly lie about how they just aspire to be heroes and wanted to spar with Saitama. Genos tries to get them to leave, but Saitama believes them and decides to train them. He forces them to do his special workout, at which point they're too scared to stop and tell him the truth.