The narrator wishes the reader farewell. He compares the reading process to a journey in which he and the reader are fellow passengers in a coach. He hopes that he has been an entertaining companion.
Partridge visits Jones at the prison to break the horrifying news that Mrs. Waters is Tom's mother. Tom receives a letter from Mrs. Waters in which she alludes to this fact and says she has been greatly affected by it. She adds as a postscript that Fitzpatrick is on the recovery. Black George arrives next at the prison, and he and Tom exchange warm greetings. Black George reports that Squire Western and Mrs. Western have had a vicious argument that has concluded with Mrs. Western declaring she never wants to see her brother again. The Squire has been reconciled with Sophia, however. The narrator retraces his footsteps to describe how this reunion came about. Sophia took her father's side when arguing with her aunt about Lord Fellamar—this delighted Squire Western and endeared Sophia to him once more.
Allworthy visits Nightingale's father and, after three hours, convinces the old man to see his son. On his entrance to the house, Allworthy spots Black George, but takes no notice of him. Later, he asks Nightingale's father what business he had with Black George. Nightingale's father shows him five bank bills of one hundred pounds each that Black George has given him to invest. Allworthy recognizes the bills as those he gave to Tom.
Mrs. Miller is depressed about Tom's situation, but Allworthy cheers her somewhat by telling her he has no doubt there will soon be a reconciliation between Nightingale and his father. Allworthy summons Dowling from Blifil's room to ask him what should be done about the case of the bank bills. Mrs. Miller interrupts their conversation and introduces Allworthy to Nightingale, who brings the tidings that Fitzpatrick has recovered and admitted provoking the duel. Mrs. Miller urges Nightingale to remind Allworthy in what great esteem Tom holds him. Tears come to Allworthy's eyes and he reminisces briefly about the time he discovered the infant Tom between his sheets. The narrator hints that Allworthy's tears have been partly caused by a letter that he received from Square.
The narrator presents Square's letter to Allworthy. Square writes that he is terminally ill, and that he has been reflecting on his past behavior. He feels worst about his behavior to Tom, who is innocent of the crime for which Allworthy condemned him. In fact, during Allworthy's illness, Tom was the only person who showed any real concern and compassion. Tom's mirth was motivated by Allworthy's recovery. Square hints at the dark designs of "another Person." The narrator also presents a letter from Mr. Thwackum to Allworthy, in which Thwackum haughtily and arrogantly tells Allworthy to consider him for the position of Vicar of Aldergrove if the current vicar should die.
Mrs. Miller tells Allworthy that Nightingale discovered that the men who accused Tom were commissioned to do so by a Lord who wanted Tom sent off on a ship. Nightingale also happened to see Mr. Dowling with these men in the tavern. Shocked, Allworthy calls for Dowling, but he has already left. Allworthy asks Blifil if he knows whether Dowling has seen the eyewitnesses of Tom's duel. Blifil does not speak for some moments, which leads Mrs. Miller to shout "Guilty!" Allworthy asks Blifil why it is taking him so long to answer. Blifil answers that he sent Dowling to mollify the evidence of the witnesses. Allworthy now feels even more tenderness for Blifil. He proposes that they all pay a visit to Tom in prison. Partridge arrives and privately tells Mrs. Miller that Mrs. Waters is Tom's mother. Allworthy, hearing that the man with Mrs. Miller is Tom's servant, summons him. He immediately recognizes him to be Partridge. Surprised, he asks if he is indeed Tom's servant. Allworthy asks Partridge many questions about Jones.
Allworthy asks Partridge why he has been serving his own son. Partridge tells Allworthy that he is not actually Tom's father. He tells Allworthy what has happened in his life since he was found guilty. First Partridge worked for a lawyer in Salisbury. Then he moved to Lymington, where he worked for a lawyer for three years, after which he set up a school. One day, one of his pigs broke into his neighbor's yard and Partridge was taken to court. Allworthy tells him to get to the point. After seven years in the Winchester jail, Partridge taught at Cork in Ireland. He then moved to Bristol, where he met Tom. Partridge now tells Allworthy that Mrs. Waters, with whom Tom has had a relationship, is Tom's own mother. As Allworthy expresses his horror at the situation, Mrs. Waters walks in and asks to talk to Allworthy alone.
Mrs. Waters tells Allworthy the story of Tom's conception and birth: his father, Mr. Summer, was the son of a clergyman whom Allworthy raised and even sent to the university. Mrs. Waters is not Tom's mother, although she did put the baby Tom in Allworthy's bed. She reveals that Bridget Allworthy, Allworthy's own sister, was Tom's mother. After Allworthy left for London, Bridget approached Jenny's mother and confided her secret in her. Together they contrived to send Deborah Wilkins, the maid, to Dorsetshire to have her out of the way. Allworthy is shocked that his sister did not tell him the truth. Jenny exculpates her, however, by saying that she intended to tell Allworthy one day. Bringing the conversation back to the present, Mrs. Waters tells Allworthy that Dowling approached her and promised her money from a "very worthy Gentleman" if she continued her prosecution of Tom. Allworthy guesses that this gentleman must be Blifil.
Squire Western arrives. He has discovered Sophia's letters from Tom. Allworthy offers to speak to Sophia after he has spoken to Dowling. Once Western has left, Jenny tells Allworthy that she spent twelve years with a man who swore to marry her but never actually did. She fled to Captain Waters for protection, and lived with him for many years under the alibi of being his wife. She met Tom when Captain Waters left to oppose the Jacobite rebels.
Mrs. Waters falls to her knees and praises Tom's goodness in saving her. Dowling interrupts them. Motioning to Jenny, Allworthy asks Dowling if he knows "this Lady." Dowling has to admit that he does. Allworthy now carries out a kind of trial by which he finds out that Blifil was indeed responsible for trying to bring further prosecution against Tom. Allworthy asks how Dowling could have been Blifil's accomplice. Dowling confesses that he already knows that Tom is Allworthy's nephew—on her deathbed, Bridget Allworthy took Dowling's hand and bid him tell Allworthy that Tom was her son. She also wrote a letter to Blifil explaining the story. Dowling entrusted the letter and story to Blifil, who promised to pass on the information to Allworthy.
Mrs. Miller returns and Allworthy tells her the shocking news. Mrs. Miller is overjoyed that Tom has been proven innocent. Before she leaves, Mrs. Waters tells the company that Tom will soon be released from prison. Allworthy summons Blifil and tells him to produce the letter that Bridget wanted him to deliver to Allworthy. Blifil's situation is "to be envied only be a Man who is just going to be hanged."
Allworthy reads Tom's letter to Sophia. The beauty of it brings tears to his eyes. Allworthy visits Sophia and congratulates her on her refusal to marry Blifil, which shows foresight on her part. Allworthy says that he has a different proposal for her—he has another nephew, whom he would like her to marry. Sophia expresses surprise at never having met this mysterious nephew, and Allworthy tells her that it is Tom. Sophia says she can appreciate that Tom must be a worthy nephew, but she cannot accept him as a husband. Squire Western suddenly bursts in and chastises Sophia. In his country dialect, Western bellows that he has a letter from Lady Bellaston relating that Tom is out of prison and on the loose. Western warns Sophia to stay away from the man. Allworthy takes this opportunity to acquaint Western with recent events. Squire Western now begs Allworthy to bring Tom to court Sophia that afternoon.
Allworthy apologizes to Tom for his past behavior. Tom says that there is no need for retribution; the joy he is experiencing now atones for his suffering. Tom laments his follies and vices, but Allworthy brushes them away, praising Tom for not being a hypocrite. Allworthy tells Tom that he has visited Sophia, and urges Tom to submit to Sophia's will. Mrs. Miller meets with Tom and tells him that she has explained to Sophia that Tom's proposal letter to Lady Bellaston was not meant seriously. Sophia still complained that Tom was a "Libertine," but Mrs. Miller told her that Tom turned down Mrs. Hunt. Mr. Western arrives, extremely impatient for the afternoon courtship festivities.
Jones tells Allworthy and Mrs. Miller how he gained his liberty from the prison. Mrs. Waters assured Fitzpatrick that Tom did not have an affair with his wife and, consequently, Fitzpatrick admitted that he initiated the duel. Moreover, Fitzpatrick is so delighted with what Mrs. Waters has told him that he praises Tom to Lord Fellamar, who decides that he should assist this man whom he affronted by his advances to Sophia.
Allworthy wishes to punish Blifil, but Tom argues for forgiveness. Mrs. Miller and Allworthy want Blifil to leave the house as soon as possible. Tom asks that he may be the messenger of the news. He finds Blifil bawling on his bed, although Blifil is frightened rather than contrite. Tom tells Blifil the news—he comforts Blifil and offers to provide for him. Blifil thanks Tom profusely, then departs. Allworthy reveals Black George's corruption to Tom. Tom tells Allworthy of Black George's generosity to him while he was in prison, but Allworthy is determined to punish Black George for his dishonesty. Partridge and Tom are reunited.
Tom meets Sophia at Western's house. They are both finely dressed and look breathtaking. At first they remain silent. Sophia suggests that Tom judge his own behavior—she tells him that only time will prove whether he can cast aside his wild desires. She does not understand how he could have been unchaste in Upton. Tom argues that the delicacy of women prevents them from imagining how sordid men can be. He argues that amours of the body do not affect the amour of the heart. Sophia accepts his reply, but says that she will only marry him after twelve months. They kiss. Mr. Western bursts in and, after teasing the lovers with bawdy jokes, orders Sophia to marry Tom immediately. Sophia says that she cannot disobey her father. Western looks forward to having a grandson in nine months.
The wedding is filled with mirth, and those who were unhappy before are happy now. The narrator summarizes the future. Tom makes Allworthy agree to give Blifil an annuity of 200 pounds, even though Allworthy refuses to speak to Blifil. Blifil converts to Methodism in the hopes of marrying a rich Methodist widow who lives near to him. Mrs. Fitzpatrick separates from Fitzpatrick. Mrs. Waters marries Parson Supple, and Allworthy grants her an annuity of sixty pounds. Partridge sets up a school with the help of Tom. He is engaged to Molly Seagrim. Sophia and Tom now live on Western's estate and have two children, a boy and a girl. Western has retired to a smaller estate, but visits the couple frequently. Tom has conquered his cheeky streak. He and Sophia are still very much in love, and hold each other in the highest esteem. They show kindness and respect to all around them.
Book XVIII follows the archetypal comic finale in that it consists of the resolution of a series of misunderstandings: Square's letter dispels Blifil's false accusations, and Mrs. Waters's testimony reveals Tom's true parentage.
The summary of future events that concludes the novel is typical of Romantic comedy, and it serves to show which characters have experienced a "Revolution," and which ones have not. Blifil, for example, shows no contrition about his wicked acts and instead begins plotting afresh. It is appropriate that Tom, the protagonist, has undergone the greatest transformation—he now lives in perfect chastity with Sophia as his wife. Tom's forgiveness of Blifil makes Tom a better man even than Allworthy, who wishes to punish Blifil. In such a way, rather than simply being born good, Tom achieves the status of "hero" by the novel's end. The vast arc that Tom makes from beggarly bastard to wealthy, dignified gentleman makes the novel a kind of Bildungsroman"—that is, a novel that charters the growth of a single character from infancy to maturity.