Sister Mary, a 20-year-old girl with a fascination for cowboy stories, is sent to a new church in a remote town in the middle of nowhere. As she arrives at her new church, she is welcomed by a parish church consisting of friendly Father DuPont, devoted Mother Carmen, and socially anxious Sister Penelope.
Her welcoming is soon interrupted as a violent gang led by hired gun Harrison Lee attacks the church, threatening them to leave the church within a week. The next day, Father DuPont and Sister Mary go to town and explain their problem to Sheriff Wilbur, who suggests they move to town while things calm down.
As Sister Mary questions about what option could they use to defend themselves, they get in the way of a bank robbery conducted by brothers Jebediah Kinski and Sergio Almada. Jebediah takes Sister Mary hostage. Sheriff Wilbur prepares a group to find them and he receives help from candidate to mayor Brandon Baxter.
The brothers take Sister Mary to their hideout. As Sister Mary confronts them for their sinful life, Jebediah tries to gag her, but Sister Mary bites him and tries to escape with the money. However, since they are hiding in the middle of the desert, she has no choice but to stay with them. They come to an agreement. She won't turn them in as long as they teach her to shoot.
The brothers teach her to shoot. As he gets to know her, Jebediah falls in love with her. Jebediah takes her back to the church and gives her his gun so she can fight them back, if required. Sister Mary returns to her church.
As the night falls, the church is once again attacked by Harrison Lee's gang and much to everyone's surprise, Sister Mary uses Jebediah's gun to fight back, shooting a warning shot at Harrison Lee. Father DuPont tells Mary that the way of violence isn't the right path and that God will protect them, and Sister Mary confronts him, expressing her religious doubts. Meanwhile, Harrison Lee reunites with Brandon Baxter, explaining to him about Sister Mary's defense actions. Baxter asks Harrison to shoot Sister Mary if necessary as he needs Harrison to infuse fear among the people so he can be voted mayor.
The next day, Sergio arrives at the church, searching for Jebediah. Worried that he might have been arrested, Sister Mary goes to town and finds that Jebediah has indeed been arrested. She and Sergio come up with a plan to release him. Sister Mary visits Jebediah in jail, explaining to Sheriff Wilbur that the brothers didn't harm her and leaves a Bible for Jebediah. Jebediah escapes shortly after using a hair clip hidden in the Bible.
Sister Mary returns to the church and finds an angry Mother Carmen, who confronts her about if she wants to be a nun. Sister Mary expresses her doubts about the calling and they agree that the best for her might be to leave. As Sister Mary prepares to leave the church, Harrison Lee's gang attacks once again. Sister Penelope convinces Mother Carmen and Father DuPont to help Sister Mary fight the gang. As the fight escalates, Jebediah and Sergio arrive and help.
Eventually, Harrison Lee decides to use dynamite and blows up the church. Conflicted about what he has done, Harrison Lee curses the nuns for not leaving before when Sister Mary comes up from the ruins and shoots him. Sheriff Wilbur\'s searching party arrives and Harrison Lee comes clean, giving up on the fight and explaining Baxter's plan. Baxter refuses to accept his involvement. He claims that Sister Mary is working with Jebediah and Sergio and that they all should be arrested but Sheriff Wilbur decides to arrest Baxter. As Baxter's bodyguard is about to shoot Sheriff Wilbur, Jebediah saves him. Jebediah and Sergio return the money and Sheriff Wilbur agrees to let them go.
Mary, no longer a Sister, leaves the church, explaining that maybe she can help God by following a different path, and leaves together with Jebediah and Sergio for a new adventure.
