Pisa, 11th century. Spina, a young priestess daughter of Peter, is devoted to keeping a flame burning that illuminates the way back for those returning from the Crusade. But the Florentine knight Lamberto falls in love with her and the two young men fail in the vow of chastity that Spina committed. The shame for having betrayed the promise of purity made pushes Spina to suicide and Lamberto kills himself with her.