A single man arose to face the evil threatening humankind.
His name was Saitama. He became a hero for fun. Three years of special training made him lose his hair, but he gained invincible power. He achieved such strength that he can defeat any opponent, however strong, with one punch. Unable to make use of his full strength, he is unsatisfied as a hero, but a cyborg named Genos asked to become his pupil! Then the House of Evolution takes an interest in Saitama's power and sends monstrous assassins after him...