Tito Melema arrives in the Florence of the Medicis penniless and unknown, but the sale of some rare jewels in his possession soon brings him into the circle of the wealthy, learned men of the city, among them Bardo, a blind antiquarian. Bardo is a great scholar who continues his annotations of Greek and Roman books through the eyes of his beautiful daughter, Romola. Bardo’s only interests in life are his library and his museum, and he has reared his daughter in innocence of the outside world. Bardo accepts Tito eagerly, for he is always glad to meet a scholar who has traveled widely. He tells Tito of a son whom he had lost.

Tito’s fortune has at last come to him with the sale of all his jewels except a single ring. He recalls that the money properly belongs to Baldassare Calvo, the man who has been almost a father to him and who might now be a slave in the hands of the Turks. If Baldassare is alive, Tito tells himself, he will spend the money for the old man’s ransom, but he is not sure his foster father still lives.

Tito quickly entrenches himself in the learned society of Florence. While sitting at a window with a friend during the yearly festival of San Giovanni, patron saint of Florence, Tito fancies that a monk in the crowd below is looking at him with malice. Also glancing up at Tito from below is the beautiful Tessa, daughter of a milk vendor, whom Tito had met on the day of his arrival in Florence.

Later, as he walks through the crowded streets, Tito rescues Tessa from some jostling revelers. When he leaves her, he meets the strange monk he had seen gazing at him from the crowd earlier in the afternoon. The monk, Fra Luca, gives him a note that has been brought from a pilgrim in the Near East; Tito wonders why he finds the monk’s face so familiar. The note is from Baldassare, who pleads with Tito to rescue him from slavery. Unwilling to give up his happy life in Florence, Tito ignores his foster father’s plea.

Attracted to the lovely, grave Romola, Tito spends many hours reading and writing manuscripts with her blind father. One day, when Tito has the opportunity to be alone with Romola for a moment, he declares his love to her, and Romola shyly confesses her love for him. That same day, Monna Brigida pays a call on her cousin Bardo. When she accidentally mentions the name of a Dominican monk, Dino, Tito discovers that the lost son of Bardo is not dead; rather, he has been banished from his father’s house. Tito realizes that Fra Luca is Dino, and he fears exposure of his benefactor’s slavery. He feels the time is right for him to ask the old man for permission to marry Romola; he does so, and Bardo readily consents to the marriage.

Tito learns that Fra Luca is dangerously ill at Fiesole. One evening, Romola tells him that her dying brother has sent for her. Tito fears that Fra Luca will tell her about Baldassare’s plea for help—a story that Tito hopes will die with him. In despair, he wanders through the city and accidentally meets Tessa. The two take part in a ribald mock marriage ceremony that amuses a gaping crowd, and Tito allows Tessa to believe that he has really married her. Unwilling to undeceive her, he makes her promise to keep the marriage a secret. Meanwhile, Dino dies without revealing to Romola the story of Baldassare and the ungrateful Tito. Tito and Romola are married.

Bardo dies, leaving Romola to carry on his scholarly work. Meanwhile, as the Medicis struggle to maintain control of their city, the political situation deteriorates. The Medicis’ troubles are made worse by the charismatic preaching of Savonarola, who proclaims that the impending arrival of the French is God’s will. This situation helps to advance Tito’s fortunes; he becomes an interpreter in negotiations with the French. On the day the French king arrives in the city, the soldiers lead through the streets a group of prisoners who beg their ransoms from the Florentines. The mocking mob cuts an old man loose from his fetters and allows him to escape into the crowd. The prisoner runs blindly into Tito, who is standing with a group of dignitaries. Tito turns and finds himself looking into the face of Baldassare Calvo, who then disappears into the crowd.

Fearing Baldassare’s revenge, Tito buys a coat of mail to wear under his clothes. He begs Romola to sell her father’s library and leave Florence with him, and when Romola refuses, he secretly sells the library. Betrayed by her husband, Romola flees Florence, only to be met outside the city by Savonarola, who persuades her to honor her marriage vows and return to Tito.

In his search for a place to stay, Baldassare comes by chance to the house where Tessa and her children by Tito live with a deaf old peasant woman. The deaf woman gives the old man permission to sleep in the hayloft. Tessa eagerly confides in Baldassare, telling him that Tito sent her to live with the old peasant woman, whom he pays well for the care she gives Tessa and his children, and that he has sworn the two women to secrecy. While Baldassare lies in the loft, Tito arrives to see Tessa. Suspecting from her description the identity of the old man, Tito goes to his foster father to ask his forgiveness—he has decided that Baldassare should come to live with him and share his comfort. The old man cannot forgive, however; he lunges at Tito with a knife, which breaks against the chain mail Tito is wearing. He then threatens to expose Tito and ruin him.

At a dinner in Florence, Baldassare appears to denounce Tito before his politically important friends. The trembling old man is pronounced mad, however, and is sent to prison. During a plague, the prison is emptied to make room for the sick, and Baldassare is released. He spies on Tito until he learns about Romola; he then approaches Romola and tells her of Tito’s relationship with Tessa. When Romola learns of Tito’s betrayal, she is able to piece together all the suspicions she has had about her husband—his long absences from home, his strange moods, and his secret fears. One day, she finds little Lillo, Tessa’s son, wandering lost in the streets. She takes the child to his home, and there she realizes that she has discovered Tessa.

The final blow comes to Romola when her godfather, Bernardo Del Nero, the only person in the world she still loves, is arrested for helping the Medicis in their plotting to return to Florence. Romola knows that Tito has been a spy for both political factions; he has gained his own safety by betraying others. Romola reveals to Tito her knowledge of Baldassare’s story and the truth of the old man’s accusations against him. Romola tries to prevent Bernardo’s execution by pleading with Savonarola to intervene and gain his release, but the preacher refuses. Disillusioned and sorrowful over her godfather’s death, Tito’s betrayals, and Savonarola’s falseness, Romola leaves Florence to seek a new life.

Tito also plans to flee Florence, for his double-dealing has been discovered. As a mob pursues him out of the city, he throws away his money belt, and while those in the crowd scramble for it, he jumps into the river. Weakly, he pulls himself ashore on the opposite side. There Baldassare, now a starving beggar, finds him. In a final effort, the old man flings himself onto his exhausted enemy and strangles him.

After spending many months in another city, Romola returns to Florence, where she learns of her husband’s murder at the hands of an old man who had long been his enemy. Romola understands the justice of Tito’s violent end. She finds Tessa and the children and brings them to live with her, as she is determined to repair the damage that Tito’s deceptions have wrought.