An alternative history novel set in a world in which the Protestant Reformation never took place, and in 1976, the word of the Yorkshireman Pope John XXIV, is not to be questioned. Science is not mentioned; electricity is not allowed; Catholicism is the law. Hubert Anvil is a singer in the choir of St. George's Basilica whose life is thrown into chaos when his teachers and the Church hierarchy decree that the boy's voice is too precious to sacrifice to puberty. Consequently, Hubert must undergo castration (the 'alteration' named in the title) in order to preserve his voice. Will he escape this turn of fate?