Mumen Rider vs. Deep Sea King

Mumen Rider vs. Deep Sea King was the battle between the C-Class hero Mumen Rider and Deep Sea King, a Demon-level mysterious being.

Prelude
After Genos sacrifices himself to save a little girl from Deep Sea King's acid, the monster grabs Genos by the head and throws him into a nearby wall and then quickly follows-up with a devastating punch that sends Genos through the wall and out onto the street. The monster congratulates Genos from giving him "a few scratches", but notes that they've already healed. He also mocks the cyborg's foolish move of protecting the girl, noting that it cost him the fight. Before Deep Sea King could finish off Genos, however, he is attacked from behind by Mumen Rider.

Battle
Mumen Rider performs Justice Crash on Deep Sea King, though it only serves to slightly annoy the monster. Mumen Rider then tries to punch the monster, but Deep Sea King simply grabs his arm, smashes Mumen Rider against the ground multiple times and throws him in the air. Thinking the fight was over, the monster tries to get back to kill Genos, but Mumen Rider uses Justice Tackle. This angers Deep Sea King, who then transforms into his monstrous form and swats the hero away. Mumen Rider refuses to give up, stating that despite the fact that he is too weak to be a B-Class hero and that nobody is expecting him to win, he will not back down. Encouraged by cheers from the onlooking civilians, he prepares to stand up to Deep Sea King, but the monster simply punches him in the air, defeating him and knocking him unconscious. Arriving in the nick of time, Saitama catches Mumen and congratulates him on the fight and praises his resolve.

Aftermath
Saitama takes over the fight for Mumen Rider.

After Mumen Rider recovers, he meets Saitama at an udon shop and reveals that he was the one who sent the thank-you letter. He also congratulates Saitama for his promotion to the B-Class and offers to treat Saitama to a meal.