Place: A convent near Siena.
Time: The latter part of the 17th century.
The opera opens with scenes showing typical aspects of life in the convent — all the sisters sing hymns, the Monitor scolds two lay-sisters, everyone gathers for recreation in the courtyard. The sisters rejoice because, as the Mistress of Novices explains, this is the first of three evenings that occur each year when the setting sun strikes the fountain to turn its water golden. This event causes the sisters to remember a sister who has died, Bianca Rosa. Sister Genevieve suggests they pour some of the golden water onto her tomb.
The nuns then discuss their desires — while the Monitor believes that any desire at all is wrong, Sister Genevieve confesses that she wishes to see lambs again because she used to be a shepherdess when she was a girl, and Sister Dolcina wishes for something good to eat. Sister Angelica claims to have no desires, but as soon as she says so, the nuns begin gossiping — Sister Angelica has lied, because her true desire is to hear from her wealthy, noble family, whom she has not heard from in seven years. The rumors have it that she was sent to the convent in punishment.
The conversation is interrupted by the Infirmary Sister, who begs Sister Angelica to make a herbal remedy — Sister Angelica's specialty. Two tourières then arrive, bringing supplies to the convent, as well as news that a grand coach is waiting outside the convent. Sister Angelica immediately becomes nervous and upset, thinking rightly that someone in her family has come to visit her. The Abbess chastises Sister Angelica for her inappropriate excitement and then goes on to announce the visitor, the Princess, Sister Angelica's aunt.
The Princess explains that Angelica's sister is to be married and that Angelica must sign a document renouncing her claim to her inheritance. Angelica replies that she has repented for her sin, but there is one thing she cannot offer in sacrifice to the Virgin — she cannot forget the memory of her illegitimate son who was taken from her seven years ago. The Princess refuses to speak, but finally informs Sister Angelica that her son died of fever two years ago. Sister Angelica, devastated, signs the document and collapses in tears. The Princess leaves.
Sister Angelica is seized by a heavenly vision — she believes she hears her son calling for her to meet him in paradise. She makes herself a poison and drinks it, but realizes that in committing suicide, she has committed a mortal sin and has damned herself to eternal separation from her son. She begs the Virgin Mary for mercy and, as she dies, she sees a miracle: the Virgin Mary appears, along with Sister Angelica's son, who runs to embrace her.