The narrator provides that his purpose in the text will be to explore "human nature." As such, his story veers between several extremes - comedy and tragedy, low and high society, moral and base.
Squire Allworthy, a man defined by his interminable kindness, returns to his Somersetshire estate to find a child abandoned in his bed. He gives the child to his sister Bridget to look after, and they investigate to determine that the child's mother is a young woman named Jenny Jones. She leaves the area, and Allworthy decides to raise the boy, Tom Jones. Tom is brought up alongside Allworthy’s nephew Blifil, Bridget's son. They are educated by two men of differing outlook, Thwackum and Square. Blifil is a miserable and jealous boy.
Tom is an impetuous character who supports his friend, the poor gamekeeper Black George Seagrim, even when that support causes him trouble. Meanwhile, through his relationship with Squire Western, Allworthy's boorish but affable neighbor, Tom slowly falls in love with the squire's daughter Sophia, who also comes to love him.
However, Tom cannot pursue Sophia because his girlfriend Molly, daughter to Black George, grows pregnant with what he believes to be his son. When he is revealed not to be the father of Molly’s child, Tom is free to pursue his emerging love for Sophia.
Blifil conspires against Tom, and he is unjustly turned out of Allworthy’s house and away from Sophia. Further, because Tom is a bastard child, Squire Western refuses to support Tom's suit for Sophia, and instead wishes her to marry Blifil so that he can consolidate their lands. Sophia hates Blifil, and is tortured by her father's cruel insistence.
Allworthy gives Tom a fair sum of money to support himself, but it is stolen by Black George. Tom considers joining the military. He meets up with Partridge, a teacher-cum-barber whose reputation was ruined when he was believed to be Tom’s father years before. Partridge initially believes that he can return to Allworthy’s favor if he reunites the man with Tom, but Partridge ultimately becomes a devoted companion along the way. Tom frequently shows his benevolent spirit by helping an unsuccessful highwayman, a beggar and a lady in distress - all gestures which are richly repaid later in the novel.
Sophia is locked up for refusing to marry Blifil. She flees, and both Tom and Sophia try to locate each other on their respective journeys to London. She discovers he has slept with Mrs. Waters (a woman he rescues) and that he is mentioning her name to strangers, and she decides he must not love her. She then heads to London, and Tom follows her.
While in London, Tom takes up with the promiscuous and wily Lady Bellaston, with whom Sophia is staying. She promises to help him but endeavors to keep the lovers apart.
Sophia is also roughly courted by Lord Fellamar. Her aunt, Lady Western, is anxious for her to marry him, whereas her father is still adamant that she will marry Blifil. Sophia decides she will marry no-one without her father’s consent, but will also not be told whom to marry.
Tom is innocently caught up in a duel and imprisoned. His friend Nightingale, loyal companion Partridge, and devoted landlady Mrs. Miller investigate the course of Tom’s imprisonment and sustain his contact with Sophia. There is tension when it is initially believed that Mrs. Waters is Tom’s mother, but this is revealed to be untrue. Allworthy is shocked to discover that Tom is his nephew, Bridget’s illegitimate but first-born son, and that Blifil has known about this since his mother’s death. It is discovered that Blifil engineered Tom's imprisonment to get him out of the way.
The charges against Tom are dropped and his marriage to Sophia is blessed by both Allworthy and Squire Western. Blifil is banished but has an annuity from Allworthy and Tom. Sophia and Tom live happily, close to Nightingale and Mrs. Miller’s daughter Nancy, whose union Tom facilitated. Partridge is given an annuity to start a new school and marries Tom’s first girl, Molly Seagrim.