For centuries, the world (or, actually, Tokyo in specific) has suffered a plague of Yamiko. These beings are born from the dark side of the human heart, and are utterly depraved and without morals. Fortunately they often move quickly into their own mirror world, but visits from that world to create more are common, and when still in the 'Light' world they can cause horrible damage.
Stronger and tougher than a human, with no morals to limit them, Magnificent Kamen empowers and trains 'Sailors' to fight a secret war to keep them in check. Sailor Salvation is a thankless job, and one with a high turnover, because Sailors don't get the benefit of extra healing or toughness in this world. They're normal girls who can banish demonic beings, and it tends to be a very lethal job after too many years, not to mention hard on a tender teenager's mind to see the horrible things they do. It is, in one sentence, a darker and more brutal Magical Girl setting.