Earth is saved by heroes for now, but the large-scale battle between the Hero Association and the Monster Association is far from over. The dangerous hero hunter Garou is still on the loose despite the efforts to capture him, and the Monster Association has abducted a notable figure, leaving the Hero Association in turmoil. With a strict deadline for the rescue, the heroes set out in search of the monsters' hideout.
Though he is heavily injured, Garou is recruited by the Monster Association in the hope of making him a part of the leadership. In order to ascend the ranks, the hero hunter must first prove that he is ready to give up on his humanity by killing a hero. It does not take long for Garou to find his victim, but that very person is the oblivious Saitama, who effortlessly knocks him out. This humiliating defeat causes Garou to be branded as a traitor by the monsters. Moreover, the next clash between the two associations approaches rapidly. As both sides are ready to engage in any tactics and use any pawns at their disposal, the safety of the world is at their mercy.