Mumen Rider vs. Deep Sea King is the battle between the top Ranked C-Class hero Mumen Rider and the Demon level threat the Deep Sea King.
Prologue
Mumen Rider uses Justice Crash
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. Before the Deep Sea King could finish off Genos, he is attacked from behind by Mumen Rider.
Battle
Deep Sea King smashing Mumen Rider on the ground
Mumen Rider performs Justice Crash on the Deep Sea King, though it annoyed the monster it had no effect on him. Mumen Rider then tries to punch the monster, but the Deep Sea King simply grabs his arm and 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 being cheered on by the civilians
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, but the monster simply punches him in the air and Mumen Rider was caught by Saitama, who praised 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.