Ep 5. The Ultimate Master

Saitama decides to train under a highly skilled martial artist named Bang, who is also known as the Silver Fang. Bang trains Saitama in his martial art style, which he calls "Water Stream Rock Smashing Fist." Saitama proves to be an excellent student, quickly learning the moves and mastering the style.

Meanwhile, the Hero Association receives a distress call from a group of heroes who are under attack by a monster named Melzargard. One of the heroes, a cyborg named Genos, heads to the location to battle the monster, but he is quickly defeated.

As the other heroes try to come up with a plan to defeat Melzargard, Saitama receives word of the battle and decides to go help. He arrives on the scene and quickly defeats Melzargard with one punch. However, Melzargard's body starts to regenerate itself, and Saitama realizes that he must destroy all of the monster's cores in order to defeat him for good.

Saitama heads inside Melzargard's body and starts destroying the cores, but he is soon confronted by another of the monster's minions, Gouketsu. Gouketsu challenges Saitama to a fight, but Saitama is able to defeat him with one punch as well.

After defeating Gouketsu, Saitama continues to destroy the cores, finally defeating Melzargard and putting an end to the threat.

Back at the Hero Association HQ, the other heroes are amazed at Saitama's incredible feat, and he is promoted to Class A. However, Saitama is unimpressed, feeling that the promotion is meaningless to him. He heads out of the building, ready to take on his next challenge.