One-Punch Man Episode 6: The Terrifying City

Five days after becoming a registered C-Class hero, Saitama learns from Genos that inactive heroes get dropped from the registry after a week, so with only a day left, he heads out on patrol in his old neighborhood but finds no action.

Speed-o'-Sound Sonic reappears, skulking around and scaring residents to draw Saitama into a fight; he unleashes exploding shurikens on the street, causing panic among bystanders. A C-Class hero named Tanktop Tiger shows up, mistakes Saitama for trouble amid the chaos, and tries to hype himself up with the crowd before Sonic's attack knocks him out, confirming Sonic as the villain. Saitama, now motivated, sneaks behind Sonic and knocks him out with a single powerful hand chop, ending the threat effortlessly.

Meanwhile, at Hero Association headquarters, a green-haired woman in a black dress named Tatsumaki demands to investigate rumors in the abandoned "ghost town" part of City Z but is dismissed since it's not an S-Class matter; instead, A-Class heroes Spring Mustachio and Golden Ball are dispatched. City Z is depicted as a monster-plagued wasteland with warning signs, empty streets, rampant destruction, and even monsters avoiding it due to its terrifying reputation.

Genos detects the heroes' arrival near Saitama's apartment building while they're patrolling but they move on. Spring Mustachio and Golden Ball spot a seaweed-based monster called Kombu Infinity (also referred to as Kelpton or Komu Infinity), who came from out of town seeking strong monsters to join but decides to claim the area himself. Golden Ball fires a slingshot projectile, which Kombu dodges before counterattacking; Spring Mustachio extends his sword for a strike, but Kombu evades and weakens him. Mustachio calls for reinforcements as he's knocked out.

Saitama, returning from shopping, encounters the battered Kombu, who attacks him but is swiftly killed by Saitama in an off-screen clash, with Saitama later seen making kombu soup from the remains. A group of other heroes later inspects the fight site, marveling at the destruction and speculating about more monsters.

In the post-credits scene, Genos reports Saitama's hero rank rose from 388 to 342, but Saitama grows jealous upon learning Genos ranked 6th in a popularity poll.