Michael is an atheist. So as he walks through the doors at St. Clare's—a strict Catholic school—sporting a plaid tie, things can't get much worse. His dad has just made the family move again, and Michael needs a friend. When a girl challenges their teacher in class, Michael thinks he might have found one, and a fellow nonbeliever at that. Only this girl, Lucy, is not just Catholic . . . she wants to be a priest.

But Lucy introduces Michael to other St. Clare's outcasts, and he officially joins Heretics Anonymous, where he can be an atheist, Lucy can be an outspoken feminist, Avi can be Jewish and gay, Max can wear whatever he wants, and Eden can practice paganism. After an incident in theology class, Michael encourages the Heretics to go from secret society to rebels intent on exposing the school's hypocrisies. When Michael takes one mission too far—putting the other Heretics at risk—he must decide whether to fight for his own freedom, or rely on faith, whatever that means, in God, his friends, or himself.