Since the Fall of Rome the Holy Catholic Empire has governed the Earth without opposition, its military arm under the banner of the Cross and Cudgel ruthlessly spreading the Gospel of Jesus Christ to all the peoples of the world and mercilessly eradicating all other religions that threatened to rise up. All spiritual and temporal power is centered in the Vatican, where the pope's authority is absolute, and the emperor merely serves as a puppet monarch without any real sovereignty.
But something happens to shake the Church's tenets of faith to their foundations. The planet is visited by a space ship from an alternate Earth where Catholicism is merely one of a multitude of faiths practiced by humanity, and there is no unifying empire but rather a myriad of independent countries all competing with each other for position and planetary resources. To the prelates of the Holy Catholic Empire this other Earth is a world of chaos and heresy gone wild. One cardinal, a confidant of the pope, is given the assignment of visiting this dangerous new world. His mission: to decide if this alternate Earth is a new revelation in God's plan or a source of contagion for his own world that must be wiped out.