France, a holding cell. A young woman in knight's armor sits on the stone floor, holding her hand against a candle flame, as a second prisoner is thrown into the room. They are Jeanne Darc and Jean Moulin, two of the greatest heroes of French history, both killed for doing the same thing at different points in time: uniting the disparate and dispirited remains of an army into a force strong enough to overthrow an occupation.

But the joy they find in their similarities snaps quickly under the weight of their differences. They discover each advocates a worldview the other hates: Jeanne's fierce belief in God and Heaven lead her to value sacrifice, even to the point of death, while Jean's life has been spent making up for the deaths of others, and living without regard for religious ideas of the afterlife. Both decide being forced to watch a great life wasted, Jeanne's in death and Jean's in damnation, is the one torture neither will suffer without a fight.

As Jean's torture becomes more intense and Jeanne's pyre grows higher, the holding cell becomes an arena as two of history's most passionate heroes run the gamut of emotions in a debate that risks damning their souls and damaging their legacies. Communion of Saints uses this clash to explore the differences between living and dying, between waste and sacrifice.