Mumen Rider vs. Deep Sea King

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

Prologue
After the Deep Sea King takes advantage of Genos' weakened state, 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 the Deep Sea King could finish off Genos, however, he is attacked from behind by Mumen Rider.

Battle
Mumen Rider performs Justice Crash on the Deep Sea King, though it only serves to annoy the monster. Mumen Rider then tries to punch the monster, but the Deep Sea King simply grabs his arm, smashes Mumen Rider against the ground and throws him in the air. Thinking the fight was over, the monster tries to finish off Genos, but Mumen Rider uses Justice Tackle. This angers the Sea King, who then transform 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 praises his resolve.

Aftermath
After Mumen Rider recovered, he met Saitama at an udon shop and revealed that he was the one who sent the thank you letter. He also congratulated Saitama for his promotion to B-Class and offered to treat Saitama to a meal.