WARNING: The following contains spoilers for One-Punch Man Vol. 21, by ONE, Yusuke Murata, John Werry and James Gaubatz, now available in English from Viz Media.
The Hero Association was rocked to its core by the revelation their usual monster foes banded together to form the Monster Association, a group of creatures ranging in size and ability lurking under the sewers beneath Z-City. Shortly after surfacing, the monsters kidnapped Waganma, the son of the Hero Association's wealthy benefactor, making his rescue a top priority. While Saitama ventures underground on his own, the S-Class Heroes step up to remind everyone why they've earned their organization's top rank as the battle against the monsters escalates into an all-out underground war.
Confident in their upgraded abilities and home field advantage, the monsters have the heroes head-on, with Rhino Wrestler leading the charge. He faces off against the S-Class Hero Atomic Samurai who remains completely unflappable in the face of his hulking opponent before slicing through him effortlessly with the blade of grass in his mouth before turning his attention to the other assembled monsters. Meanwhile, the ninja warrior Flashy Flash confronts two rival ninjas that had since consumed to monster cells to transform into upgraded warriors serving the Monster Association. Coming from the same clan as Speed-o-Sound Sonic, it's clear the three have a history ,but Flashy Flash quickly dispatches them before they can threaten the rest of the Hero Association, dismissing the ease in which he kills them both to a lack of training on their part.
As the monsters and heroes continue to fight in the sewers, the unassuming boy genius and S-Class Hero Child Emperor ventures deeper into the Monster Association's headquarters alone. Using the vast array of gadgets and weaponry stored in his backpack, the hero is able to plow through any and all monsters that cross his path without so much as a second thought, including the villainous Phoenix Man who had led the coordinated attacks on the surface and drafted Garou into the Monster Association. Child Emperor shrugs off Phoenix Man's assault, kills him and rescues Waganma after gassing his captors.
However, while Waganma has been freed from his cell, he and Child Emperor have a long way to go before reaching the safety of the surface world and Hero Association as they face increased increased resistance from the monsters. As the two make a break for the surface, they encounter Phoenix Man who has been resurrected just like his mythological namesake and is significantly more powerful than during his first confrontation against Child Emperor. With even his most elaborate defenses now overpowered, Child Emperor unleashes his ultimate weapon as he pilots a gigantic mech suit to fight the upgraded Phoenix Man directly.
One-Punch Man Vol. 21 ends with its titular protagonist nowhere to be seen, presumably still trudging through the dark to reach the battle between the Hero and Monster Associations, likely to arrive right at the moment when he's needed the most. Saitama might not be coming alone -- an epilogue chapter shows Charanko and Suiryu both recovering from their previous battles and working to get back into fighting shape in the hospital, with Suiryu visibly impressing the nurses. The Monster Association may not be giving up Waganma so easily, but the heroes still have more tricks of their own to unleash.