Ambrosio, the celebrated monk of Madrid, is beginning to grow conceited. He congratulates himself on his popularity as a confessor and speaker. He has deserved this - who but him has come through youth unsullied by any temptation? Religion cannot boast his equal.
Though the most beautiful women of Madrid flock to him as their confessor, he remains pure. His gaze lingers on a beautiful painting of the Virgin Mary, and he praises her delicate lips, her flowing hair, and her lovely eyes. He considers bartering thirty years of virtue for a mere taste of such lips, but instantly dismisses the idea. Only a painting can express such beauty, never a real woman. Besides, this is the Virgin Mary - Ambrosio reasons that he is attracted to her divinity, not her humanity.
Rosario, a young novice at the monastery, interrupts his reverie. Rosario is a mysterious figure: he is quite quiet about his origins and generally envelops himself in the thick cloths of his cowl. However, glimpses of the face from under this covering suggest remarkable loveliness. Rosario is also extremely serious in his religious observances, rivaling Ambrosio in his austerity. Ambrosio dotes on Rosario, treating him like a son. Rosario comes to Ambrosio asks for prayers for a sick friend, which Ambrosio graciously gives him. As an expression of gratitude, Rosario arranges a basket of flowers in Ambrosio's cell. Rosario hints that he is experiencing great sins and sorrows, but flees to attend vespers prayers before Ambrosio can inquire further.
Ambrosio hears the confessions of the nuns. As the last nun is leaving, however, a letter falls from her pocket and Ambrosio picks it up despite her protestations. The letter is from the nun's lover, and reveals the fact that she is pregnant and planning to escape from the convent. Ambrosio is incensed. The nun - Agnes, Lorenzo's sister - falls to her knees and begs Ambrosio for mercy. She attempts to explain the situation: she fell in love with Raymond long before she took her vows, and became a nun only because she thought he was unattainable. They met nightly in the convent garden, and in one unguarded moment she ended up pregnant. Agnes begs Ambrosio to overlook her error, particularly for the sake of her unborn child; the penances of the convent of Saint Clare are severe, and may result in her losing the baby.
Ambrosio is infuriated with her weakness, and insists that it is necessary to give the letter to the Prioress in order to place Agnes back on the righteous path. The Prioress is even more furious, particularly because it was Ambrosio (whom she admires greatly) that made this discovery about one of her charges. Agnes is hauled away by the Prioress, but she screams a parting curse at Ambrosio, saying that one day he too will commit an infraction of the flesh, and he will endure the same lack of mercy that he showed Agnes. Ambrosio is somewhat chilled by her words, but reasons that his virtue is proof against all temptation.
To settle his mind, Ambrosio wanders around the monastery garden. There he finds Rosario, deep in despair, wishing that he could live entirely separate from all mankind. Ambrosio attempts to counsel him, expounding upon the joys of living in community, as the monks in their monastery do. Rosario replies hat it is this very monastic community that troubles him and makes him wish for exile; he hints at terrible transgressions that leave Ambrosio puzzled.
At last, Rosario comments that it was on nights like this that his sister Matilda spent her last unhappy months. He explains that Matilda fell in love with a married man named Julian, and she pretended to be a servant in order to gain entrance into his house as the maid of Julian's wife. Her devoted service earned her Julian's respect, but eventually her love for him grew so great that she had no choice but to confess it. Julian was horrified by her deception, and kicked her out of the house. Devastated at the loss of her beloved, Matilda died from a broken heart. Ambrosio cannot help but feel sympathy for this poor girl, and he says so to Rosario. Rosario replies, "Do you pity her? Oh father! Then pity me!" Ambrosio starts, but Rosario explains that at least his sister had a friend (Rosario himself), whereas he is entirely alone in his misery.
Ambrosio again asks Rosario what troubles him so very much, and Rosario forces Ambrosio to promise that he will not expel the novice monk from the monastery until he has completed his novitiate. Puzzled, Ambrosio makes this promise. At last, with great emotion, Rosario reveals he first of his secrets - he is a woman.
Ambrosio is stunned, and attempts to run away from Rosario. She seizes him, however, and continues her confession - she is Matilda, the girl who resorted to deception to be near the man she loved, and Ambrosio is her beloved.
Ambrosio is so shocked that he cannot move or speak, and Rosario/Matilda takes advantage of his confusion to provide her backstory. She is the daughter of a noble family, but her father died young and her uncle brought her up. This uncle insisted on offering her the finest possible education while also emphasizing the merits of religion, so Matilda grew up with a strong, intelligent, and pious mind. She was unimpressed by the cowardly, lusty youths who sought her hand in marriage; only Ambrosio captured her heart, and so she decided to pose as a novice monk to be near him. She did not lust for him; she basked in the glory of his being. Inspired by him, she took on great penances and eventually became one of the most promising novice monks.
Listening to this story, Ambrosio experiences a variety of emotions: surprise, confusion, resentment, and conscientiousness in forming a gentle reply. He also feels self-satisfied that a young, wealthy, and beautiful woman would renounce the world out of love for him. Additionally, he is experiencing a strong undertow of sexual desire. At length, he replies to Matilda, explaining that he cannot allow a woman to remain in a community of celibate men. She points out the promise her made her as well as her excellent record of self-control. She is no ordinary weak woman, she insists, and she deserves to stay at the monastery. Ambrosio disagrees: he thinks it will be unhealthy for her to stay around him, and she must leave tomorrow. Ambrosio is unmoved by her desperate pleas.
At last, Matilda draws a dagger and places it at her breast, explaining that she will kill herself rather than leave the monastery. The dagger tears open her robes, exposing her cleavage, and Ambrosio stares at her exposed breast. Moved partly by compassion and partly by lust, he says that he will allow Matilda to stay.
Later, in his cell alone, Ambrosio reflects on this bizarre turn of events. He ponders his reasons for allowing Matilda to stay: how flattered he feels at having unconsciously won her heart, and his sense of sorrow at losing a valued companion. He reasons that he will be able to forget Matilda's sex and resume their former relationship. His mind wanders again to the beauty of her breast, and he hastily kneels in prayer before the image of the Madonna. He falls into an uneasy sleep, troubled by a number of erotic dreams.
Upon waking, Ambrosio realizes that his kindness toward Matilda was in fact motivated by greed and desire; he decides that she must leave the monastery if they are both to remain pure.
He meets her once more in the gardens, and gently but firmly declares that they can never meet again - Matilda must leave the monastery in three days' time. Matilda protests desperately, but Ambrosio explains that if she stays, he fears becoming the victim of desires he has resisted for thirty years. Matilda submits to his will and says she will enter a convent run by a female relation, but first she asks for a token of Ambrosio's affection to carry with her. She asks him to pick a nearby rose for her. Ambrosio does so, but jumps back suddenly - a serpent, hidden among the flowers, has bitten him. Ambrosio quickly loses consciousness.
Matilda's hysterical, inconsolable screams bring quick help. Ambrosio is carried back to his cell, and the doctor of the monastery examines his wound. The bite has already swelled Ambrosio's hand to a ghastly size, and the discolored ooze seeping from it indicates that was a bite from one of the most poisonous snakes in the world - a Cientipedoro. The doctor declares that Ambrosio will die from the venom in three days' time. The only cure would be to extract the poison, but the monks have no tool with which to accomplish this. Ambrosio is wracked by a violent fever, and Rosario/Matilda alone stays by his side during the night.
The next day, an incredible change has taken place. The swelling of Ambrosio's wound has decreased dramatically, and blue-colored pus no longer leaks from it. The amazed and delighted monks declare this a miracle - Saint Francis has surely saved their Abbot. At last, the excited monks are sent away from Ambrosio's sickbed, and he is left alone with Rosario/Matilda. Matilda seems to be an oddly happy mood, and she plays him a song about unrequited love on her harp. Her beautiful voice charms Ambrosio, but he is even more overcome when her cowl falls away from her face as she concentrates upon her instrument, revealing a pair of lovely coral lips. Her sleeve, pulled up, reveals an equally delicate white arm. Struggling with desire, he closes his eyes and pretends to sleep.
Matilda gazes down at the monk, and thinking that he is asleep, indulges in a monologue about her love for him. She voices her joy at having this moment to gaze upon him without his knowledge and without the risk of tempting him to sin. She also reflects on her deep sorrow at the thought of their impending separation, and her terror at his recent illness. Ambrosio marvels at the purity of her love.
Matilda turns her attentions to the painting of the Virgin Mary that hangs in Ambrosio's cell, remarking upon her beauty and remarking (as Ambrosio himself did at the beginning of the chapter) that Ambrosio's fixation on the painting must surely be out of interest in the divinity, not the woman. Matilda hopes that the monk will show her a tiny fraction of the attention he has shown the portrait as she lies on her deathbed. Shocked, Ambrosio jolts himself out of bed, asking Matilda why she thinks she will be on her deathbed so soon. Matilda is startled and her cowl falls away from her face - revealing a countenance that exactly resembles the portrait of the Virgin Mary that Ambrosio had gazed at so longingly.
Shyly, she explains that after she fell in love with him, she commissioned a portrait of herself as the Virgin Mary, and one of her emissaries (a Jewish merchant) made sure that Ambrosio bought it in the market. She delighted in the fact that Ambrosio was so fixated on her portrait, but she was determined to keep their love pure, so she concealed her face and her sex, and established a platonic friendship with him. Deeply touched by her affections (and filled with desire at the idea that his beloved painting has taken in a woman's form), Ambrosio reminds Matilda that they must part forever in three days. With a truly dreadful expression, Matilda says that they will indeed part forever, and then rushes from the room.
Ambrosio is torn apart by conflicting desires. He reasons that it is nobler to endure temptation daily and overcome it than to exile temptation altogether, and he considers allowing Matilda to stay in the monastery. His medical condition steadily improves, though he is still troubled by erotic dreams at night. The next day, Ambrosio and Matilda meet again in the garden. He tells her that she can stay in the monastery as long as she wants, but she replies that they will soon be separated by death. Alarmed, Ambrosio notes her pale skin and sunken eyes, and calls the doctor immediately, though Matilda refuses any medical care. Ambrosio is struck with terror at the thought of losing his dear companion.
At two in the morning, a monk rushes to Ambrosio's cell and tells him that Rosario (the monks do not know her true identity) has been poisoned and may die soon. Ambrosio hurriedly runs to Matilda's cell, telling the other monks to leave them alone, and demands to know what has happened. Matilda calmly explains that she sucked the poison form Ambrosio's snakebite in order to save his life, and now she is dying from that same poison. Ambrosio's last defenses are washed away with awe that this woman would sacrifice her life for his. Matilda explains that this last trial has instilled in her a more earthly desire for him, and that she must "enjoy" him before she dies. In the solitude of the cell, the two come together in sexual union.
The great Ambrosio, the treasure of Madrid, is actually quite the narcissist. He has conquered the minor sins of lust, but he is totally consumed by the greater sin of pride. The unsettling lust he exhibits toward his painting of the Virgin Mary shows that he has still not fully mastered his passions, but has only given the appearance of having done so. This makes him even more dangerous - a famous monk ruled by dark passions, he is like a wolf in sheep's clothing. Still, it is true that he has committed no sins in his life.
The character of Ambrosio emerges as a counterpoint to the lusty but good-hearted Lorenzo. Both men are prominent members of Madrid society; but whereas Lorenzo is open and honest in his dealings with people, Ambrosio is closeted. Likewise, Lorenzo is humble and Ambrosio is secretly consumed by pride. The more worldly man (Lorenzo) emerges as the more likable and sympathetic, a theme that will continue throughout the book.
Ambrosio's total lack of compassion for Agnes, who grovels at his feet begging mercy for her unborn child, is yet more proof of a spirit that is callous rather than filled with grace. Agnes' parting words to Ambrosio (her prediction that he too will one day endure temptations and will be punishes for them unmercifully) foreshadow some great transgression.
Matilda and Agnes are parallel cases of two women begging Ambrosio for mercy. One receives his help and the other does not - the deciding factor between the two is Matilda's use of her sexuality to catch Ambrosio's attention. Her breast, needlessly exposed as she holds a dagger to her heart, arouses Ambrosio's lust as well as his pity, and he promises to keep her secret.
When Ambrosio finally confronts Matilda and explains that she needs to leave the monastery and go to a convent. Something peculiar happens. As he plucks a rose to offer Matilda as a token, he is bitten by a serpent and becomes seriously ill. The association of a serpent and a beautiful woman in a garden is evocative of the Garden of Eden, in which Eve and the Serpent tempted Adam to eat the forbidden fruit and fall from grace. Ambrosio also has a fall from grace when he breaks his vows of celibacy to sleep with Matilda.
Matilda's actions (the exposed breast, the serpent under the rose, the portrait of herself as the Madonna) all point to a character who is much more cunning and manipulative than she lets on. Even her words of devotion to Ambrosio may just be part of her plan.