The Good Soldier opens with John Dowell, the narrator, informing the reader that "this is the saddest story [he] ever heard." His story, he explains will be about the nine-year acquaintance that he and his wife Florence had with another couple, Edward and Leonora Ashburnham. Because this novel is comprised of Dowell's thoughts as he writes them, the plot is not chronological; it shifts back and forth between past events and Dowell's current reflections on those events. Overall, the novel is divided into four parts.
In Part I of The Good Soldier, Dowell explains the background of the four protagonists. He and Florence are Americans, from Philadelphia and Stamford, Connecticut, respectively. Edward and Leonora are British, the former a Catholic from Ireland, and the latter an Anglican from a respected family in Fordingbridge. Dowell sees similarities between the two couples. Both Dowell's wife, Florence, and Edward Ashburnham are "heart patients." The couples meet in Nauheim, a health spa, in August 1904 where both "invalids" come to improve their health. The four strike up a friendship. Dowell describes Edward as perfectly honest and upright, someone with whom you could trust your wife. Dowell goes on to narrate in detail an excursion that the two couples took to a town which houses Martin Luther's protest. In the bedroom, while Florence is discussing the Protest, Leonora realizes Florence and Edward will have an affair. Dowell is blind to this implication. In this part of the novel, Dowell compares their friendship to a minuet, and to "an extraordinarily safe castle," but then he bemoans the end of permanence and stability now that their friendship, nine years later, is over.
Edward has been having numerous affairs: with a woman in the back of a railway car, with the mistress of a Grand Duke in Monte Carlo, with a woman married to an army officer in India, and with a young, submissive woman the Ashburnhams have brought with them from India to Nauheim. After the affair with La Dolciquita, Edward brought the Ashburnhams into deep debt, and Leonora took complete control of all his money and land. The Ashburnhams have been in India eight years to save money, and now that they are in Nauheim, Leonora feels she has regained control of their lives. On the day that they meet the Dowells, Leonora sees Maisie Maidan coming out of her husband's bedroom and she assumes they are having an affair. She boxes Mrs. Maidan's ears, and Florence sees this act of violence. Leonora, who cares deeply about propriety, attempts to befriend Florence to cover up what has been witnessed; that is how the two couples become friends.
Part I ends with Mrs. Maidan's death. Now that Florence has started an affair with Edward, Mrs. Maidan overhears them talking about her, and finds out that Leonora has brought her along so that she may be a "safe" mistress for Edward. Mrs. Maidan is appalled and dies; unlike Florence and Edward, she suffers from a real heart condition. Leonora lives with the guilt over this death.
In Part II, Dowell explains Florence's real desire to be the lady of Fordingbridge, in the town in England where her ancestors were forced to live centuries ago. Edward Ashburnham's estate, Branshaw Manor, is in Fordingbridge. The chapter flashes back to Dowell and Florence's first meeting in Connecticut. They were married against the wishes of Florence's aunts, the Misses Hurlbird, and immediately left for Europe. On the boat trip to Europe, Florence feigned a heart condition, and her pretend heart condition keeps them forever trapped on the European continent. While Dowell and Florence lived in Paris, Florence has an affair with a low-class artist named Jimmy, but Dowell knows nothing about it. Dowell's own marriage to Florence has remained unconsummated for thirteen years.
After Florence's nine-year affair with Edward, she plans to divorce Dowell and marry Edward. But one night, August 4, 1913, Florence sees Edward in the park alone at night with his young ward, Nancy. She grows insanely jealous and runs back to the hotel, only to find an old acquaintance, Mr. Bagshawe, telling Dowell about Florence's indecent relationship with Jimmy many years ago. Part II ends with the discovery of Florence's dead body; she has poisoned herself.
Part III opens with Dowell considering marriage to Nancy, the Ashburnhams" young ward. But Edward, too, is in love with her. Nancy, who has recently left convent school, admires her uncle immensely for his heroic deeds and she assumes he and Leonora have a healthy marriage; she is not in love with him in the same way. After Florence's death, Leonora, Edward, and Nancy all return to Branshaw Manor in England. There, Leonora has a nervous breakdown, since she surmises that Edward is in love with Nancy. In this section, Dowell narrates the entire history of Edward and Leonora's marriage. Through all his affairs, Leonora always hoped that Edward would return to her and love her again. But when Edward began his affair with Florence, Leonora's love died.
In Part IV, Leonora realizes that Nancy is safe from Edward. She sees that the small amount of sexual morality that her husband possesses will keep him from starting an affair with Nancy. She has a breakdown and tells Nancy what a terrible husband Edward is. Nancy's ideals are crushed, and her eyes are opened to the realities of the world. Dowell says that together, Leonora and Nancy begin to slowly torture of Edward. He suffers greatly and plans for Nancy to travel to India to live with her father. She sends a telegram from India saying she is having a good time. Edward is crushed; all he desires is to have her worship him from afar. Edward commits suicide by stabbing himself with a penknife. Leonora remarries a "perfectly normal" man named Rodney Bayham and they have a child. Nancy goes mad; the only word she can say is "shuttlecock." Dowell buys Branshaw Manor and becomes Nancy's permanent caretaker.