Ninjas Arc

The Ninjas Arc is the eighteenth arc of the series.

Plot
Flashy Flash, determined to make Saitama his disciple to help him unleash his full potential, visits Saitama's apartment where Saitama is playing video games with King. After a brief scuffle with Genos and multiple rebuffs from Saitama, Flash's persistence is finally rewarded when he convinces Saitama to have a training bout at one of Flash's training sites. If Flash defeats him, Saitama will become his disciple, but if Saitama can land even one hit in thirty minutes, Flash will have to leave him alone. As their match goes on, Flash is shocked by Saitama's ability to keep up so casually. Saitama easily breaks Flash's sword and is about to land a punch on the cornered ninja, but Genos interrupts the match before there can be a conclusion, telling them several Demon-level Mysterious Beings have appeared nearby and that they should go help stop them.

Meanwhile, Heavy Kong, Bone, and Peach Terry are struggling to defeat the Internet Surfers. As they fight, the Internet Surfers notice Speed-o'-Sound Sonic watching nearby. Saitama, Genos, and Flashy Flash then arrive on the scene. Sonic is surprised to see Saitama hanging out with Flash, and a stand-off between the heroes, the monsters, and Sonic occurs. In a flash of speed, the Internet Surfers have been defeated and Saitama has buried Sonic in the concrete.

Later, Saitama asks Flashy Flash about his and Sonic's relationship, so Flash tells him their backstory. He explains the existence of the Ninja Village and the inhumane things that he, Sonic, and the rest of their classmates had to endure in the brutal and oppressive training there. Once Flash became strong enough, he decided to destroy the hellish place by murdering all of the staff and his fellow classmates in a bloody massacre, and poisoning Sonic so as to not have to kill him directly. However, Sonic evidently managed to survive.

Elsewhere, Sonic is resting in his hideout and easing his injuries when he is suddenly approached by Gale Wind and Hellfire Flame, alongside a bevy of other ninja graduates from the Ninja Village calling themselves the Heavenly Ninja Party. They explain that the leader of the Ninja Village has finally awoken after being put into a coma fifteen years ago by a battle with Blast, and it is now time to get revenge on both Blast and Flashy Flash. They then forcibly enlist Sonic to help them find Flashy Flash.

The next day, Sonic challenges Flashy Flash to a duel and the two meet in a forest. Flash is reluctant to fight his former friend, but Sonic is ready to kill him. Meanwhile, Saitama talks to Genos about going to challenge Sonic and taking one of his swords in the process, so he can give it to Flash to make up for the one he broke during their match earlier. Back in the forest, Flash is surprised by how much Sonic has grown but is still able to outclass him - however, they are interrupted by the Heavenly Ninja Party suddenly arriving all at once. They move to execute Flashy Flash but Sonic stops them, much to their irritation, expressing that he wants to kill Flash himself. Sonic fights back against the Heavenly Ninja Party, with Flash soon joining him, and together they defeat them all before returning to their duel.