The young Allegra di Vitali bored with life in her guardian's Venetian palazzo until the day sexual curiosity draws her to look at the depraved illustrations he keeps in his private chamber. Awakened to womanhood she tries to deny her new passion for the flesh by submitting to the life of a nun.

The strange order of the Convent of Santa Agatha provides new tests and temptations, encouraging her to perform ritual acts with men and women who inhabit the cloistered world. As the Unhallowed Rites unfold, Allegra undergoes bizarre ceremonies before finally confessing to her own ravenous sexuality.