Lolita is a novel narrated by Humbert Humbert, a literature scholar born in 1910 in Paris, France, who is obsessed by what he refers to as 'nymphets' (defined as sexually desirable girls between the ages of nine and fourteen). This obsession with young girls appears to have been a result of his failure to consummate an affair with a childhood seaside sweetheart, Annabel Leigh, before her premature death from typhus. Shortly before the start of World War II, Humbert leaves Paris for New York. In 1947 he moves to Ramsdale, a small New England town, to write. One of the rooms he's considering renting is in the home of Charlotte Haze, a widow, who appears to be sexually interested in him, offering him an "ominously low" rate. As the two make their way through Mrs. Haze's tour of the house, Humbert rehearses different ways of turning her down, but then, being led out into the garden, spies Haze's twelve-year-old daughter Dolores (variously referred to in the novel as Dolly, Lolita, Lola, Lo, L), sunbathing in the garden. Humbert, seeing the Annabel Leigh in her, is instantly attracted to the daughter, and eagerly agrees to rent the room.

Charlotte becomes his unwitting pawn in his quest to make Lolita a part of his living fantasy. When Mrs. Haze drives Lolita off to summer camp, she leaves an ultimatum for Humbert, saying that he must marry her (for she has fallen madly in love with him) or move out. Humbert chooses the former for the sole reason of making Lolita his stepdaughter, intending to use heavy sedatives on both her and her mother so he can molest Lolita in her sleep, although we never learn specifically what he plans to do.

Humbert starts to write a diary recording his life in Ramsdale, and more specifically his relationship with Lolita. He locks the diary in a drawer. While Humbert is in town and Lolita is away at camp, Charlotte (who expresses a morbid jealousy of, and interest in, her new husband's past love life) manages to open the drawer and finds his diary, which details his lack of interest in Charlotte and impassioned lust for her daughter. Horrified and humiliated, Charlotte decides to flee with her daughter. Before doing so, she writes three letters—to Humbert, Lolita, and a strict boarding school for young ladies to which she apparently intended to send her daughter. Charlotte confronts Humbert when he returns home. Retreating to the kitchen, he tells her that the diary entries are just notes for a novel. But Charlotte has already bolted from the house to post the letters. Crossing the street, she is struck and killed by a passing motorist. A child retrieves the letters and gives them to Humbert, who destroys them.

Humbert picks Lolita up from camp, telling her that her mother is desperately ill in a hospital, and takes her to The Enchanted Hunters, a hotel of regional repute (where he meets a strange man who seems to know who he is, later revealed to be Quilty), intending to use the sleeping pills on her. They have little effect on her, however. She instead seduces Humbert (the first of only two times she is recorded as doing so)—and he discovers that he is not her first lover, as she had a sexual affair at summer camp with the camp mistress' son. After leaving the hotel, Humbert tells the now-troublesome Lolita that her mother is dead. Alone and frightened, Lolita has no choice but to accept Humbert into her life on his terms. Driving Lolita around the country in Charlotte's car, moving from state to state and motel to motel, Humbert bribes Lolita for sexual favours. Eventually they settle down in another New England town, with Humbert posing as Lolita's father and Lolita enrolled in a private girls' school where the headmistress sees Humbert's possessive supervision as that of a strict old-world European parent.

Humbert nevertheless is persuaded to allow Lolita to take part in a school theatrical club (extracting additional sexual favours from her in exchange for his permission). Ominously, the title of the play—The Enchanted Hunters—is similar to the name of the hotel where they technically became lovers. Lolita is enthusiastic about the play and is said to have impressed the playwright, who attended a rehearsal, but before opening night she and Humbert have a ferocious argument and she bolts from the house. Found by Humbert a few minutes later, Lolita declares that she wants to immediately leave town and resume their travels. Humbert is delighted, but increasingly guarded as they again drive westward, nagged by a feeling that they are being followed and that Lolita knows who the follower is. He is right: Clare Quilty, an acquaintance of Charlotte's, nephew to the local dentist in Ramsdale, and the author of the play being performed at Lolita's school, himself a pedophile and amateur pornographer, is tailing the couple in accordance with a secret plan of escape devised together with Lolita. While Humbert becomes increasingly paranoid about being tailed, Lolita becomes ill and recuperates in a nearby hospital. One night she checks out with her "uncle", who has paid the hospital bill. Humbert, still clueless as to the identity of Lolita's abductor, makes farcical and frantic attempts to find them by inspecting various motel-register aliases, laced by Quilty with insults and jokes flavored with literary allusions.

During this period Humbert has a chaotic, two-year love-affair with a petite alcoholic named Rita, who at thirty is ten years younger than himself and a passable physical substitute for Lolita. By 1952 Humbert has settled down as a scholar at a small academic institute. One day he receives a letter from Lolita, now 17, who tells him that she is married, pregnant, and in desperate need of funds. Armed with a gun, Humbert, still driving Charlotte's car, sees Lolita again. She tells him that her husband, a nearly deaf war-veteran and the father of her unborn child, was not her abductor. Humbert offers to give Lolita his entire financial worth if she will reveal his identity. Lolita complies, saying that she really loved Quilty but the affair ended when he threw her out after she refused to perform in a pornographic film that he was making.

Leaving Lolita forever, Humbert surprises Quilty at his mansion. Quilty begins to go insane when he sees Humbert's gun. After a mutually exhausting struggle for it, Quilty, now fully mad with fear, merely responds politely as Humbert shoots him repeatedly. He finally dies with a comical lack of interest, expressing his slight concern with an affected English accent. Humbert is exhausted and disoriented. Arrested for murder, he writes the book he entitles Lolita, or The Confessions of a White Widowed Male, while awaiting trial. Upon finishing, he dies of coronary thrombosis. He is thus unaware that Lolita leaves with her husband to the remote Northwest where she too dies, during childbirth, on Christmas Day, 1952.