Saint-Preux, a young Swiss man with unusual talents and sensibilities, is accepted by Madame d’Étange as a tutor for her daughter Julie and Julie’s cousin Claire. For a year under Saint-Preux’s instruction, the women make excellent progress, until Claire goes away to visit her own family. During her absence, Saint-Preux reveals his love for Julie. After some solicitation, Julie admits that she, too, is hopelessly in love. The young people view their situation as desperate, for the Baron d’Étange, Julie’s father, has promised to marry her to his friend, de Wolmar. In addition, the baron is a lineage-proud man who would never hear of his daughter’s marriage to a commoner such as Saint-Preux, regardless of the latter’s abilities.

Julie fears that she may fall victim to her love for Saint-Preux; she writes to Claire and asks her to return as a protector. She writes to her cousin because she is afraid that, if her mother suspected the truth, she would immediately send the young man away. Claire returns, and for a time the romance continues to blossom. At last, Claire and Julie decide that Saint-Preux ought to leave until the baron returns from an absence that has kept him from home for well over a year. The women fear that the baron may dismiss Saint-Preux unless steps are taken to pave the way for the young man to continue as their tutor. Saint-Preux leaves. The women show themselves off to the baron when he returns, and he is so pleased with the progress of their education that he has Saint-Preux recalled.

Once again, the love between Saint-Preux and Julie grows. In spite of her virtue, however, Julie falls victim to Saint-Preux’s pleas and becomes his mistress. A short time later, Saint-Preux is dismissed to facilitate Julie’s marriage to de Wolmar. The shock of seeing her lover depart and the news from her father that her marriage day is not far off make Julie very ill. Only a visit from Saint-Preux, smuggled into the sickroom by Claire, saves the young woman’s life.

After Julie recovers, there follow more than a year of surreptitious meetings between the lovers. As her passion waxes, Julie’s fear of her father grows less, until she even has Saint-Preux stay with her through the night. Neither of the young people believes that they are committing sin, for they honestly feel that they are already married in the eyes of heaven and that only the baron’s attitude keeps them from living together publicly and with outward virtue.

In the meantime, both Julie and Saint-Preux have met Lord Edward Bomston, a British peer living in Switzerland. Saint-Preux and Lord Bomston become friends, even though Lord Bomston is himself seeking Julie’s hand in marriage. Bomston fails in his suit. One night, while he and Saint-Preux are drinking, Bomston charges that someone has already found Julie’s favor. Saint-Preux challenges Lord Bomston to a duel, but Julie, mindful of her reputation, sends a letter to Lord Bomston, telling him about Saint-Preux and herself and warning him that her fate and Saint-Preux’s rest in his hands. She knows that Saint-Preux would be killed if the duel were to go forward and that the duel would provoke enough scandal to ruin her and drive her to suicide. Lord Bomston is moved by her plea, calls off the duel, and publicly apologizes to Saint-Preux. Again, the two men become the firmest of friends.

Shortly afterward, Lord Bomston, interceding on Saint-Preux’s behalf with Baron d’Étange, urges the baron to permit a marriage between Julie and Saint-Preux. The baron refuses, vowing that he will never break his promise to de Wolmar and that, in any case, he will never permit Julie to marry an adventurer. Lord Bomston therefore proposes that Julie and Saint-Preux elope to England and spend the rest of their lives as his pensioners on his estate in Oxfordshire. Julie, however, absolutely refuses to leave her home without her father’s consent.

In the meantime, Claire has married a man friendly with both Lord Bomston and Saint-Preux. The tutor is forced to leave Julie’s vicinity after her father refuses to permit their marriage. Through Claire’s husband, however, the two lovers manage to maintain a correspondence.

Saint-Preux, after spending some months in France and England, returns to Switzerland to find that Julie is about to marry de Wolmar. He is so overcome that Lord Bomston spirits him away to England and arranges for him to embark with an expedition leaving from England to travel around the world. Meanwhile, Julie reconciles herself to her father’s will. Her mother, who might have permitted a marriage to Saint-Preux, has recently died.

Four years pass before Saint-Preux returns to Europe. By that time, Julie and her husband have two children and are settled into domestic tranquillity. De Wolmar, eager to see his wife happy, invites Saint-Preux to visit their home. During the visit, it becomes obvious that the two former lovers have become more or less reconciled to their situation. Both seem so filled with virtue that de Wolmar requests Saint-Preux to remain as tutor to his children. Saint-Preux, anxious to please everyone and to be near Julie, agrees to take on the responsibility, providing that Lord Bomston does not need his services elsewhere. Saint-Preux feels that he can never adequately repay the Englishman for keeping him from crime, madness, and possible death at the time of Julie’s marriage.

It turns out that Lord Bomston does need Saint-Preux’s aid for a short time. The Englishman is traveling to Italy, where he has hopes of marrying a marchioness, and he wishes Saint-Preux’s aid in the affair. Saint-Preux, however, shows Lord Bomston that the woman is vicious and prevents the marriage; he also prevents a second attempt at marriage between the Englishman and a woman of doubtful reputation.

During Saint-Preux’s absence, Julie discovers that Claire, who was widowed some time before, is in love with Saint-Preux. Hoping to help both Claire and Saint-Preux find happiness, Julie writes to the tutor and tells him of Claire’s love. Saint-Preux replies that although he esteems Claire, he cannot marry her, for he still loves Julie. Julie still hopes to arrange the match upon Saint-Preux’s return from Italy.

Before his return, however, an accident occurrs. One day, while Julie and her family are walking alongside a lake, Julie’s little boy falls in the water. In saving him from death, Julie suffers severe shock and exhaustion. The results are fatal to her. Before dying, however, she writes a letter to Saint-Preux and asks him to take over the education of her children and of Claire’s children. Her cousin, writes Julie, will take her own place in making Saint-Preux’s life complete.