Episode 12

"The Strongest Hero" is the twelfth and final episode of season one of the One-Punch Man anime series.

Summary
Boros continues to fight with Saitama, which leads to Boros losing an arm. The fight continues on, even reaching the point where Saitama finds himself on the Moon, but soon returns. Meanwhile, Mumen Rider, Stinger and Lightning Max rescue the last of the survivors. The Heroes fight Melzargard, who is ultimately destroyed by Bang, who recovers and destroys the final marble in Melzargard's final head.



The fight between Saitama and Boros continues and it is revealed that Boros has regenerative powers. However, the fight continues on to the point where they are both forced to use their most powerful moves respectively. Saitama emerges victorious with Boros admitting he was not even close to Saitama. After the fight, Drive Knight leaves after telling Genos that Metal Knight is his enemy. Sweet Mask appears and blames the S-Class heroes for the destruction of the city, nearly causing a fight which is interrupted by Metal Knight who emerges to salvage parts of the ship.



Meanwhile, Superalloy Darkshine discovers alien survivors, whom Sweet Mask makes quick work of as Genos sees his past self in Sweet Mask. Saitama emerges from the ship to witness Tatsumaki having a tantrum. Genos tells her to shut up while calling her a brat, which causes her to smash him into a nearby rock. Bang interrupts before it can get any worse. The parts Metal Knight salvages from the ship are used to rebuild the Hero Association HQ, which takes the place of City A, which is wiped off the map. The A and S-Class heroes are invited to stay in the newly-rebuilt Hero Association HQ. In a post-credits scene, Pluton, a Dragon-level Mysterious Being, has appeared in Z-City and caused massive destruction to everything in its path. Soon after, Saitama, with a single punch, defeats the monster as he cries out that life is back to one-punch defeats.

Trivia

 * Rather than having an episode monologue at the end of the credits, a short scene is shown of an original character to the anime, Pluton attacking Y-City, only to have Saitama instantly kill him with a single punch.
 * This is the first episode not to have the opening play. Instead, The Hero!! plays during the battle between Saitama and Boros.
 * In addition, this is the only episode to have the ending theme play over the actual episode.