Dark Undergrounders

Saitama is jolted awake in his apartment when a wall bursts open and a hand seizes his head. He knocks the arm loose, but a follow-up blow sends him tumbling through the wall and down into the street, forehead bleeding on impact. Before he can recover, a creature erupts from the ground and swats him aside despite his guard, and as he stands he sees a whole horde surfacing. These are the Subterranean People, bent on conquering the world, and they close in on him.

Saitama fells one attacker, then another, before tearing through the entire mob until every underground foe lies dead around his feet. A voice interrupts him, and he turns to face the Subterranean King rising from the earth, only for his alarm clock to ring and pull him out of the dream. Sitting there stunned and deflated, he hears the real Subterranean King shouting outside, walks over, and ends him with a single kick to the face. He announces he will defend the surface, but the Subterranean People have already withdrawn below, leaving behind a white flag reading "We're Sorry." Saitama is left thoroughly let down.

Saitama dreams of an all-out war against the invading Subterranean People and slaughters them one by one. He wakes just as the dream's Subterranean King appears. The genuine Subterranean King attacks outside and Saitama kills him in one kick. The remaining underground forces retreat and surrender with an apology flag, leaving Saitama disappointed.

This early Volume 1 chapter belongs to the Saitama Introduction Arc and adapts into Episode 1. The Viz release titles it Subterraneans of Darkness. It corresponds to Chapter 4 of the webcomic and was published on June 25, 2012.