Joan, a teenage country girl, shows up at the castle of Vaucouleurs. She's determined to kick the English out of France and to crown the Dauphin (that's a title for the oldest son of a king of France), Charles, as King. Joan has heard voices from God telling her that this is her destiny. Through sheer confidence and natural charisma, she manages to sway the skeptical Captain Robert de Baudricourt. He gives her soldier's clothes, armor, and other supplies to assist in getting to the Dauphin.
Upon arriving at Charles's court, Joan wins over most everybody. First, she's able to pick Charles out of a crowd, which some view as a miracle. Her humility and reverence for the Church get the Archbishop on her side. Then of course, there's the Dauphin himself. It takes a little doing, but after a good old fashioned pep talk she convinces him to stop messing around and stand up for France and himself. Charles grants her control of the army.
She's off to Orleans, a town under siege by the English. Joan meets Dunois, the leader of the French troops at Orleans. He has been waiting for a while for the wind to change. It's the only way he can sail his soldiers up the river and launch a sneak attack on the English. When the wind switches directions upon Joan's arrival, Dunois is convinced that Joan has been sent by God. They march off together, to liberate Orleans.
Meanwhile, Joan's enemies are plotting against her. The Earl of Warwick and the Chaplain de Stogumber, both Englishmen, meet with Peter Cauchon, the Bishop of Beauvais. Warwick wants Cauchon to try Joan for heresy. The angry little Chaplain just wants her to die and die painfully. Cauchon agrees to try Joan, but refuses to be a political tool of the English. He says that he will do his best to save her soul.
Joan and company have been busy little bees. They've liberated Orleans, won a bunch of other battles, and have just crowned Charles as King in Rheims Cathedral. Joan, however, is unsatisfied. A good chunk of the country, including Paris, is still not under French control. She urges Charles, the Archbishop, and Dunois to press on and liberate the capital city. When they refuse she says she'll just do it without them. They tell her that, if she gets captured, they'll do nothing to help her escape.
Joan gets captured and put on trial for heresy. Sure enough, her "friends" do nothing to rescue her. The Bishop Cauchon, true to his word, does everything he can to try and save her. He's helped in this effort by the Inquisitor. It proves to be impossible, though, because Joan's personal beliefs just don't jibe with the Church's. She thinks God's messengers speak to her directly. They think God's voice on Earth is the Church and the Church alone, meaning the voices she hears must be demons. They also just can't handle with her wearing men's clothes. She absolutely refuses to dress like a woman as long as she's a soldier. In the end, they're forced to condemn her to death.
Twenty-five years later King Charles has a dream, in which Joan and good number of the other characters show up to have a chat in his royal bedroom. We learn the fate of everybody and, more importantly, we learn of Joan's legacy. King Charles now rules all of France. He set up a hearing to have her name cleared. We also learn from a time-traveling cleric that, many years afterward, Joan was made a saint by the Catholic Church. Everybody tells Joan how awesome she is and how they're sorry that they sold her out. Joan says, great, now can I come back to Earth as living person again? No way, says everybody and they all make excuses to exit the dream.
At the end of the play, Joan is left alone in a pool of light. She asks God when the world will be ready to accept saints like her.