On the planet Lithia a small team of human scientists is working to assess the alien world's suitability as a port of call for space travelers from Earth. One of the scientists, a biologist named Ramon Ruiz-Sanchez, is also a Jesuit priest and, as he studies Lithia and its dominant species, an intelligent reptile twice the height of humans, he begins to ask himself questions about the nature of Lithian society. Is Lithia the paradise it seems, a place where perfect peace and contentment reign, or is it something entirely different? Is it, in fact, a planet created by the devil specifically to tempt man with a vision of an unfallen world? As the narrative shifts back to Earth and follows the growth to adulthood of Egtverchi, a young Lithian who has been sent back with the exploratory team, Ruiz-Sanchez's doubts and crises of conscience only grow worse until he is impelled towards a dramatic and irreversible resolution of them.

Nearly fifty years after its first publication, A Case of Conscience remains one of the most unusual and thought-provoking SF novels ever written. Blish gives no indication that we are supposed to assume that the Jesuit's beliefs about Lithia are correct. His interest is not in religion as such but in the moral dilemmas that confrontation with 'otherness' produces. The conclusions at which Ruiz-Sanchez arrives are not those likely to be reached by most readers but the genuine agonies of conscience he suffers are brilliantly evoked. In his later career, Blish worked largely on Star Trek novelizations but his original fiction, from Cities in Flight (a collection of linked stories in which tramp cities, plucked from their terrestrial settings, wander the universe) to A Case of Conscience, makes philosophically stimulating and imaginative reading.