ENTER GENOS. Genos doesn’t mess around this week. He gets right down to the Sea King Clobberin’. And it looks, for a hot moment, like he was sufficient, until the Sea King swoops back into view and clobbers him right back. Genos holds out, buying valuable time for the surely en route-by-now Saitama, but when a little girl cheering him on gets targeted by the King’s acid loogie, he blocks it, at great physical cost.
All I can say is goddamn, it’s a good thing Genos is a cyborg primarily composed of replaceable artificial parts, becaue he looks near-as-makes-no-difference GONE after that acid’s done eating away at him. For a usually funny show, this is a horrific, visceral image that instills despair in that little girl.
And then, Mumen Rider tosses his bike at the Sea King. That gentle rattle of the King’s body brings the laughter right back. Mumen, unlike Saitama, belongs in Class C, at least as far as strength and ability is concerned. But he has a Class S heart, and that’s what matters as he refuses to back down and even tries to feed (in vain) off of the support of the crowd. Again, while he has no hope of victory, he’s buying time for Saitama.
ENTER SAITAMA. The man we’ve all been waiting to see saunter up to the Sea King. The King sends Saitama’s head gently, hilariously bobbling with his initial punch, but that’s all he does. Saitama doesn’t want to stand in the rain much longer, so he wants to get this over with his usual way, so he does, blasting a hole through the Sea King so hard, the force of his punch actually blows the rainclouds away, an awesome effect. It’s all over; Perfect Victory to Saitama. The crowd of evacuees vociferously voice their gratitude.
The next day Saitama and (a fully repaired!) Genos receive fan mail via HA delivery drone (they’re going to be a thing, people!), but the first letter Saitama opens is a violently scrawled accusation that’s he’s a cheat who should burn in hell. So hate mail. We’re helpfully sent back to the immediate aftermath of his defeat of Sea King. There, we see just how much one or two bad apples spoil the bunch when it comes to skewing the opinion of the whole.
One of the evacuees, whose character design seems to have been painstakingly developed to be as loathsome, adversarial, and (one!-)punchable as possible, brings up the fact that this bald guy isn’t necessarily strong, but the other heroes who fell before him were weak. He goes on to call the entire hero class system into question.
This angers Saitama, but he reacts quite differently than I expected: he embraces his role as the guy who “just” delivered the finishing blow. His self-depricating words are a means of preserving the sacrifice of the heroes before him, and he doubles down on racing in at the last second to steal wins off of them. He’s not about to let other heroes who fought with everything they had be thrown under the bus because of his mis-classification. What was left of Genos at the time manages a grin of appreciation; his master truly is amazing.
And while Saitama’s public image may not be what he might have wanted going into this whole pro hero thing, the fact is paddling against the flow of public opinion is never really going to be worthwhile as long as he’s Class C. That changes after this fight, as he becomes ranked first in C, with the option to be promoted to B after an exhaustive interview; an option he accepts, and which puts him on Amai Mask’s radar as a potential threat closing fast.
Saitama did get one hastily-scrawled letter expressing genuine thanks for his heroism. Turns out it was from Mumen Rider, who treats him to dinner at a food stand. Unlike Saitama, Mumen may be exactly where he should be—atop Class C—but that doesn’t matter to Saitama.
Mumen gave him a ride when he needed one, stood up to the Demon-class Sea King, and took an epic beating that in hindsight couldn’t have been that bad as he’s out of the hospital and ambulatory not long thereafter. We see mutual respect at that stand. And Mumen’s thanks means more to Saitama than the impersonal acceptance and love of the masses. This could be the start of a beautiful friendship.