Private eye Mike Lassiter finds himself up against a demon from Hell who prepares to eat him alive. Luckily for Mike, Sister Mary Elizabeth, a.k.a. Sister Armageddon, saves the day with her pope-sanctified, multipurpose assault rifle. She explains her mission to him: to prevent the forces of evil from bringing on the apocalypse, by whatever means necessary.

From there, Lassiter is brought along as Sister Armageddon battles demons.