Episode 9

"Unyielding Justice" is the ninth episode of the One-Punch Man anime series.

Summary
As Saitama is waiting for the Hero Association to decide whether they want him to help fight the Deep Sea King or not, Genos initiates battle, and seemingly defeats the monster with a massive attack. However, he lets his guard down and gets one of his arms ripped off. He tells the people to run while he distracts the monster, since victory is no longer assured. He seems to be doing a good job, until the Deep Sea King launches a glob of acid at a little girl, which Genos intercepts, and it melts his bionic body, nearly killing him. In that moment, right before the Sea King would have finished Genos off, Mumen Rider appears and engages the Deep Sea King. He is defeated easily, but refuses to give up. Upon seeing this, the people decide to encourage him, but to no avail. When Mumen Rider is knocked out, he is caught by Saitama, who had been instructed by the Association to provide backup and arrives in that exact moment. After exchanging a few words with the Deep Sea King, he defeats him with a single punch, and is praised by the townspeople. Sonic also reappears with his weapons in a different part of town, but assumes the Deep Sea King must have fled.

Back at their apartment, Genos and Saitama get a delivery of fan mail from the Hero Association. Genos is praised, while Saitama gets a hateful letter, calling him a cheat. The episode reverts to a flashback from right after Saitama's victory. The people are ecstatic to have been saved and are impressed by Saitama's swift victory. However, one of them starts badmouthing the other heroes, saying how they must have been frauds, no matter their rank, since they got beaten. His reasoning is that the Deep Sea King could not have been that tough if a C-Class hero defeated it in one punch. Upon hearing this, Saitama laughs, and downplays his victory to save the reputations of the other heroes by saying that he got lucky and the job was easy for him, since he came late and the monster had been weakened. He also tells the people to care for the fallen heroes, so he can continue to cash in on their work. The people accept him as nothing but a fraud, and thank the fallen for their work. Genos decides not to interfere with Saitama's decision, but promises to intervene if this choice were ever to push Saitama into a corner in the future. The episode jumps back to present time, and Saitama realizes he has another letter - this one only saying "Thank You!". He then finds a third letter from the Hero Association, telling him he is now 1st Rank C-Class, and calling him in to discuss a promotion to B-Class. Saitama accepts, making him 101st Rank, B-Class.

The episode jumps to Sweet Mask during his work. One of his subordinates tells him that Saitama moved up a rank. Sweet Mask seems to not care, as he is disappointed by the work of the A- and S-Rank heroes in the Deep Sea King case. However, he changes his mind when he is told that Saitama was the one to actually defeat the monster. Meanwhile in the hospital, Lightning Max and Stinger are recuperating, when Puri-Puri Prisoner enters, wanting to administer "Angel Shots" to the two. They escape, dropping the newspapers they had been reading, and Puri-Puri Prisoner is upset when he sees that he is called an "S-Rank Failure" in the news. The episode ends with Saitama walking home at night with his promotion. He encounters Mumen Rider in a roadside oden shop. Mumen Rider reveals he sent the thank-you letter and pays for Saitama's dinner. After the credits, Shibabawa, a fortune teller, warns an official from the Hero Association that "the Earth is doomed".

Trivia

 * The child who suddenly started loudly cheering for Mumen Rider and surprised adults around him was the one Mumen Rider helped take back his balloon when it was stuck in a tree.
 * Deep Sea King gives the episode monologue at the end of the credits.