The novel begins the day before Don Abbondio, a parish priest, has scheduled to perform marriage rites for two of his parishioners, Lorenzo and Lucia. Two agents of a local nobleman visit the priest and threaten him with death if he meets his obligations, successfully scaring him into obedience. The young man and woman try and trick the priest into being present while they covertly exchange vows, but he works out what is going on and runs away. The local nobleman’s motives in preventing the marriage of Lorenzo and Lucia are material, not romantic, in that he has wagered with his cousin that he will be able to seduce Lucia while she is still a virgin. To this end, he sends his henchmen to kidnap her, but they, too, run away when Don Abbondio discovers them and rings the church bells as an alarm. Lucia, Lorenzo, and Lucia’s mother take shelter with another clergyman, Fra Cristoforo, who arranges for Lucia to be sent to a convent in Monza to be looked after until the danger passes.

Don Rodrigo, the nobleman who tried to kidnap Lucia, enlists the help of a more powerful nobleman called “The Unnamed” so as to steal Lucia from the convent. The Unnamed discovers that the woman in whose care Lucia currently resides has had an argument with one of the other nuns and has engaged one of his men to murder her. With this information as leverage, he convinces her to send Lucia out of the convent and straight into his hands.

Lorenzo, who was sent by Fra Cristoforo to a monastery in Milan, has become caught up in civil unrest in that city concerning a shortage of food. After he drunkenly confesses his guilt for inciting a crowd to riot in the hearing of a police agent, he is arrested, though he is subsequently liberated by another mob. Fleeing from Milan to the land around Venice, he finds work with a distant relative but is soon on the run again, with the agents of Milan close behind him. The Unnamed, meanwhile, having brought Lucia to his mountain retreat, is racked with guilt once he sees how innocent she is. He refuses to hand her over to Don Rodrigo and instead seeks council from a cardinal as to what to do with her and her mother. The cardinal recommends that Lucia and her mother go to live with a noblewoman, renowned for her charity and good works, at whose home the pair will be safe from the mischief of Don Rodrigo.

A year passes, during which time Lucia and Lorenzo (who cannot return to Milan because of his exile) exchange letters, wherein Lucia admits that she promised in prayer never to marry if she was granted escape from the clutches of The Unnamed. Lorenzo returns to Milan, taking advantage of a plague that has caused chaos in the region. He finds his betrothed in the “Pesthouse” because she has caught the plague. Don Rodrigo has caught it as well and is in a far worse state than she is. Also present is Fra Cristoforo, back from the exile in which he was placed by a vengeful Rodrigo. The friar convinces Lorenzo to forgive the dying nobleman and to pray for his salvation. After being assured that her vows are not valid, since she made previous marriage vows in the presence of Don Abbondio, Lucia agrees to return to the village where she once lived with Lorenzo, a village now mostly wiped out by plague. The new nobleman in charge of their village, though the son of the now dead Don Rodrigo, proves a nice man and, incidentally, a friend of the cardinal who had arranged for Lucia’s stay with the noblewoman. The couple are married and move to the land around Venice, where Lorenzo gets work in a silk mill and Lucia has a number of healthy children.