The title character is a girl named Isabella, the daughter of Count Henrick de Vallary. After the count's wife dies, he sends his daughter to a nunnery run by her aunt, but with the provision that she will decide for herself whether or not she wants to become a nun when she turns thirteen. Despite the success of her debut, aided by her genius, piety, and beauty, Isabella turns down the lavish lifestyle she could have as the wife of a wealthy man for the life of a nun. However, she had won many suitors during her short debut, and one in particular is eighteen-year-old Villenoy. He alone of all her admirers possessed the courage to confess his feelings, but when she turns him down, he falls deathly ill. Though he does recover, his feelings remain as he heads off to war. Meanwhile, a nun named Sister Katteriena arrives at the nunnery. She is Isabella's bed-companion and closest friend. She also has a brother named Arnaldo Henault, who loves his sister dearly and comes to visit her almost every day at the nunnery grate. Isabella comes along with Katterina and, after some time, falls in love with him. She makes the decision to flee the nunnery with him, thereby, breaking her vows. As she flees, she also steals from the nunnery. Henault's father disowns him, and although they both secure pardons through the aid of Isabella's aunt, they struggle on their own in the country, failing at farming and living in poverty. In order to win the favor of his father, Henault joins the army, where he meets Villenoys. After a particularly disastrous military lost, Henault is presumed dead. Villenoy returns to console his widow and also try again to win the hand of Isabella, whom he still loves exceedingly. Impoverished, deprived of her husband, and unwilling to return to the nunnery, Isabella assents but asks him to wait three years before they marry so she may grieve for Henault. After the allotted time, they do get married, and Isabella lives that lavish lifestyle her father first told her she could have when she made her debut. However, seven years later, Henault appears at Isabella's home, not dead. He explains what happened to him, and she confesses that she had married Villenoys. She then offers him a bed in which to sleep but later suffocates him with a pillow while he sleeps, fearing that she will be shamed for having committed bigamy. Sometime after her crime, Villenoys, who was out visiting a friend, returns home. She tells him that Henault has returned but lied about her crime, saying that Henault merely died of grief after discovering that she has remarried. Villenoys decides to throw Henault's body into the river and proclaims that he will do it himself, for love of her. However, to save herself from future reproach from him, Isabella sews the canvas bag into which the body was placed to Villenoy's collar so that he will be dragged along with the body when he throws it in to the water. Her plot works, and two bodies wash up on shore some days later. Authorities identified them as Villenoys and some unknown stranger. When they brought Villenoys's corpse to Isabella, its eyes mysteriously opened, and she faints. However, due to her reputation for piety, no one suspects her. So the whole thing might have remained a secret had not for the arrival of a French gentleman, who knew Henault from the war and identified his corpse. Upon this new discovery, authorities question Isabella, who confesses immediately. She is executed, but before she dies, she gives an empowered speech about the importance of keeping one's vows, thus conquering everyone's hearts with her beauty and wisdom to the very end.