The plain-dealer, Captain Manly, returns to London after his ship had been sunk in a battle with the Dutch. He is looking for another ship because he dislikes the hypocrisy of the age and wishes to be away from the sycophancy of court and social life. Among the acquaintances who call at his quarters in London is Lord Plausible, who attempts to persuade the captain to seek his ship through influential people instead of waiting for an assignment. Manly demonstrates his love of plain dealing by showing Lord Plausible the door.

After Lord Plausible’s departure, Manly instructs the two sailors who serve him not to admit anyone to his lodgings except his ship’s lieutenant, Freeman. When Freeman arrives, he and Manly discuss the relative merits of plain dealing and hypocrisy. Freeman holds that no one can have a successful career without being hypocritical, but he cannot convince Manly that such a policy is better than telling the truth at all costs.

While they talk, Widow Blackacre forces her way past the sailors and enters Manly’s rooms. Manly makes her welcome because she is a cousin of his fiancé, Olivia. The widow, who is extremely litigious, wants Manly to appear on her behalf at a court hearing the following day. She threatens to have him subpoenaed if he does not appear. Freeman, well aware that the widow has a great deal of money, starts to court her. The widow, who has a son, Jerry, who is almost as old as Freeman, ridicules the idea because she wants to manage her own affairs and would not be able to do so if she were married.

Manly tries to find information about Olivia, whom he has entrusted with most of his fortune while he was at sea. Olivia hears of Manly’s arrival in London, but she is none too anxious to see him because she used his fortune as her own and had married Vernish, the only man Manly trusts and calls his friend. Olivia pretends to be a plain-dealer like Manly. When visited by her cousin Eliza, Lord Plausible, and others, she belabors them for their hypocrisy, saying they speak only ill of people in their absence but praise them to their faces. Her cousin reminds her that her comments about people are much worse and that she is not invited out enough in company to have an opportunity to say anything good about people to their faces.

Olivia, going on to speak plainly about Captain Manly, reveals that she does not love him and wishes to be rid of his attentions. No one present knows as yet of her secret marriage to Vernish. Manly enters her apartment unnoticed, and after the others leave, he and Olivia have words. Freeman and Manly’s page remind him to recover his money and jewels from Olivia, so Manly goes back to request them. Olivia announces to all three that she is married, though she does not say to whom, and that she cannot return the money because her husband has it.

Olivia, noticing Manly’s page, becomes infatuated and tells Manly to send the young page as messenger if they are to have any further dealings. As Manly leaves, Widow Blackacre, accompanied by her son, enter, and Freeman once more begins his suit for her hand. When she repulses him, he decides to use law instead of ordinary courtship to gain his ends.

The following morning, Manly, Freeman, and the page appear at Westminster Hall as witnesses in Widow Blackacre’s lawsuit. While away from Freeman for a time, Manly instructs his page to go to Olivia and arrange an assignation for him, for Manly has decided to get revenge by making her unknown husband a cuckold. This is a bitter errand for the page, who is actually a young woman in disguise. She had some time before fallen in love with Manly and had disguised herself as a boy to be near him.

At the court session, Freeman finds Widow Blackacre’s son and befriends him by giving him some money. The boy tells Freeman that his mother refuses to let him have any money until he comes of age. Learning that the boy has not yet appointed a guardian for himself, Freeman persuades the boy to name him as guardian, an act that transfers Widow Blackacre’s money from her hands into his. Freeman also has the boy turn over to him all the widow’s legal documents.

When Manly returns to his lodgings, his page informs him that she has succeeded in setting up an assignation with Olivia; Manly can substitute himself for the page in the darkness. When Manly hears the comments Olivia has made about him, he becomes even more furious and eager to have revenge. A little later, Widow Blackacre arrives, hoping to find Freeman and her son. When she confronts them, they tell her that she is helpless, since they have her documents and Freeman has been appointed the boy’s guardian. The widow threatens to prove that her son is illegitimate and so cannot inherit her husband’s estate.

That evening, the page goes to Olivia’s home. When Vernish appears, the page escapes without being discovered, only to return later with Manly after Olivia had sent her husband away. Manly refuses to seduce Olivia and leaves. The page is trapped when Vernish returns unexpectedly, but she escapes by disclosing herself to Vernish as a woman, incapable of cuckolding him. Vernish’s attempt to rape her is foiled by the entrance of his wife.

The page escapes through a window and returns to Manly. Later, Manly and Vernish meet. Manly is not yet aware that Vernish is Olivia’s husband, and Vernish is unaware that Manly is trying to seduce Olivia. Because they still trust each other as the best of friends, Manly tells Vernish he has been intimate with Olivia before her marriage, a fact that makes Vernish all the more certain she has cuckolded him after marriage. The page, entering during the conversation, takes Manly aside and tells him another assignation with Olivia has been set for that evening. When they part, Vernish tells himself that he will pretend to leave town and thus trap the unknown man who is seducing Olivia.

In the meantime, Freeman and several bailiffs overhear Widow Blackacre plan to use court hangers-on to prove that her son was born out of wedlock. Rather than marry Freeman and lose control of her estate, the widow finally grants an allowance to the boy and an annuity to Freeman. The lieutenant is satisfied, as the money is all he wants.

That evening, Manly and the page go to Olivia’s apartment. There, Manly overcomes Vernish in a duel. Olivia, in shame, tries to escape with the jewels and money, but Manly takes them from her. In the scuffle, the page’s wig comes off, disclosing her as a woman. Manly, impressed by her faithfulness and beauty, immediately asks her to marry him. She tells Manly she is Fidelia, heir to a large fortune. They plan to begin a new life in the West Indies.