Mr. Allworthy is a wealthy man who lives in a rural estate in Somerset, England, in the 18th century, along with his unmarried sister, Bridget Allworthy. One day, Allworthy returns from a business trip and finds an infant in his bed. He tries to find the parents and eventually concludes that the boy must be the son of Jenny Jones, who works for a local schoolmaster named Mr. Partridge. Jenny has to flee due to her ruined reputation, while Partridge is eventually accused of being the father and loses his job as a teacher. Allworthy entrusts the care of the boy to Bridget, naming the boy Tom and treating him like a member of the family.
Allworthy is friends with a man named Dr. Blifil. Dr. Blifil starts bringing around his brother, Captain Blifil, with the intention of the captain marrying Bridget and inheriting Allworthy’s considerable estate. Captain Blifil succeeds in marrying Bridget, and they have a son called Master Blifil. Before he can inherit Allworthy’s money, however, Captain Blifil goes on a walk and dies suddenly of a stroke.
Tom grows up into a generous and handsome young man, taking a particular interest in trying to help Black George, a poor gamekeeper, and his family. Tom gets in trouble when he and Black George are caught killing a bird on the property of Allworthy’s neighbor, Mr. Western, causing Black George to depend even more on Tom’s help.
Tom is attracted to Molly, Black George’s daughter, and he begins to secretly have sex with her. Around the same time, he also notices Sophia Western, who has come back to Somerset recently from a stay with her aunt, Mrs. Western. Tom gifts Sophia a bird, then later breaks his arm saving Sophia from a wild horse, and this helps Tom and Sophia realize that they love each other. But after Molly becomes pregnant, Tom tries to forget all about Sophia, feeling duty-bound to support Molly. It’s only later that Tom learns Molly has had multiple sexual partners and it’s unlikely that he is the father of her child.
When doctors proclaim that Allworthy is seriously ill when Tom and Mr. Blifil are older teens, Allworthy begins dividing up his possessions. Although he gives the majority of it to the legitimate Mr. Blifil, Tom also receives a significant portion despite being illegitimate. Eventually, it turns out that the doctors seriously overestimated the graveness of Allworthy’s disease and he makes a full recovery. Bridget, on the other hand, dies unexpectedly. An overjoyed Tom drinks a lot of wine, as he’s thrilled that Allworthy will survive. His celebrations annoy Mr. Blifil, who is mourning the death of his mother. Tom goes on to have sex in the bushes with Molly again is discovered by Mr. Blifil and the cruel teacher Mr. Thwackum, who start—and lose—a fight with Tom.
Meanwhile, Mr. Western sets his sights on Mr. Allworthy’s estate, and his sister, Mrs. Western, also wants to be part of the scheme. They realize that it could be advantageous to marry Sophia to Mr. Blifil. Although Mr. Blifil doesn’t seem passionate about this match at first, he comes around when he realizes that he could inherit Mr. Western’s property.
Sophia is devasted when she learns about this planned marriage, confessing that she loves Tom instead. Blifil and the others realize that for the marriage to happen, they’ll have to get rid of Tom. Mr. Blifil tells Allworthy about Tom supposedly attacking him earlier, and he ultimately convinces Allworthy to throw Tom out.
Tom begins to travel across England, eventually befriending a barber who happens to in fact be Partridge, the very same man who was once accused of being Tom’s father. As they travel, they stop at an inn in Upton, which Tom arrives at after rescuing a woman named Mrs. Waters from a bandit. At the inn, Tom and Mrs. Waters have sex. By sheer coincidence, Sophia has run away from home with her loyal maid, Mrs. Honour, and they are staying at the same inn. During a commotion that night at the inn, Sophia learns that Tom has been having sex with another woman. Angrily, she leaves her muff on Tom’s bed with her name on it, so that Tom will know she was there. Tom sees the muff and realizes his mistake, but it’s too late—Sophia has already left for London.
Before leaving Upton, Tom manages to obtain a pocket-book Sophia accidentally left behind with a 100-pound bank note in it. Partridge encourages the impoverished Tom to use the money, but Tom insists on bringing the money back to Sophia.
In London, Sophia stays with a relative named Lady Bellaston. Tom also makes it to London and stays with a widow named Mrs. Miller, where he wins her over by helping facilitate the marriage of her lodger, Mr. Nightingale, to her daughter, Nancy. Tom is invited to a masked ball by Lady Bellaston where he meets her and eventually starts a clandestine sexual relationship. He later regrets the relationship and decides to propose marriage to Lady Bellaston as a ploy to get her to leave him alone (it works; she’s offended). His proposal letter comes back to haunt him, however, as Sophia sees it and shuns Tom.
Tom’s fortunes take a turn for the worse when he is attacked by Mr. Fitzpatrick and wounds Mr. Fitzpatrick in a duel, seemingly mortally. Tom is imprisoned, set to be hanged if Mr. Fitzpatrick dies due to rumors that Tom instigated the argument. To make matters worse, Tom learns that Mrs. Waters is in fact the same person as Jenny Jones, meaning Tom committed incest in Upton. Eventually, Mrs. Waters reveals the truth—that she only agreed to pretend to be Tom’s mother to hide the fact that Bridget was Tom’s mother. Tom’s father was a young man staying with Allworthy named Mr. Summer, who died young of smallpox.
Mr. Fitzpatrick survives and takes responsibility for starting the fight. Meanwhile, Allworthy comes to London and learns that Mr. Blifil played a role in getting Tom imprisoned, all to try to force Sophia into marriage. Allworthy disinherits Blifil and instead gives his fortune to Tom, who he now knows is his biological nephew. Tom reconciles with Sophia and wins over Mr. Western, partly due to the fact that Tom is now a legitimate heir to Allworthy’s fortune, and Tom and Sophia marry. Tom and Sophia have children and live happily. Most of the other characters also have happy endings, except for a few like Mr. Blifil who have to face the consequences of their earlier actions.