The novel opens with a distressed letter from Lady Howard to her longtime acquaintance, the Reverend Arthur Villars, in which she reports that Mme. Duval, the grandmother of Villars' ward, Evelina Anville, intends to visit England to renew her acquaintance with her granddaughter Evelina. 18 years earlier, Mme. Duval had broken off her relationship with her daughter Caroline, Evelina's mother, and has never acknowledged Evelina. Reverend Villars fears Mme. Duval's influence could lead Evelina to a shameful fate similar to that of her mother Caroline. To keep Evelina from Mme. Duval, the Reverend lets her visit Howard Grove, Lady Howard's home, on an extended holiday. While she is there, the family learns that Lady Howard's son-in-law, naval officer Captain Mirvan, is returning to England after a 7-year absence. Desperate to join the Mirvans on their trip to London, Evelina entreats her guardian to let her attend them, promising that the visit will last only a few weeks. The Reverend reluctantly consents.
In London, Evelina's beauty and ambiguous social status attract unwanted attention and unkind speculation. Ignorant of the conventions and behaviors of 18th-century London society, she makes a series of humiliating (but humorous) faux pas that further expose her to social ridicule. She soon earns the attentions of two gentlemen: Lord Orville, a handsome and extremely eligible peer and pattern-card of modest, becoming behavior; and Sir Clement Willoughby, a baronet with duplicitous intentions. Evelina's untimely reunion with her grandmother and the Branghtons, her long-unknown extended family, along with the embarrassment their boorish, social-climbing antics cause, soon convince her that Lord Orville is completely out of reach.
The Mirvans finally return to the country, taking Evelina and Mme. Duval with them. Spurred by Evelina's greedy cousins, Mme. Duval concocts a plan to sue Sir John Belmont, Evelina's father, and force him to recognize his daughter's claim in court. The Reverend is furious. Lady Howard intervenes and manages to elicit a compromise that sees her write to Sir John, but he responds unfavorably.
Mme. Duval is furious and threatens to rush Evelina back to Paris to pursue the lawsuit. A second compromise sees Evelina return to London with her grandmother, where she is forced to spend time with her ill-bred Branghton cousins and their rowdy friends, but she is distracted by Mr. Macartney, a melancholy and direly-poor Scottish poet. At one point, she misinterprets his acquisition of pistols as a suicide attempt and bids him to look to his salvation; later she learns he had been premeditating armed robbery to change his financial status while tracing his own obscure parentage, as well as recovering from his mother's sudden death and the discovery that his beloved is actually his sister. Evelina charitably gives him her purse. Otherwise, her time with the Branghtons is uniformly mortifying: during her visit to Marylebone pleasure garden, for instance, she's attacked by a drunken sailor and rescued by prostitutes—and in this humiliating company she meets Lord Orville again! Sure that he can never respect her now, she is stunned when he seeks her out in London's unfashionable section and seems interested in renewing their acquaintance. When an insulting letter supposedly from Lord Orville devastates her and makes her believe she misperceived him, she returns home to Berry Hill and falls ill.
Slowly recuperating from her illness, Evelina agrees to accompany her neighbour, a sarcastic widow named Mrs. Selwyn, to the resort town of Clifton Heights, where she unwillingly attracts the attention of womanizer Lord Merton, on the eve of his marriage to Lord Orville's sister, Lady Louisa Larpent. Aware of Lord Orville's arrival, Evelina tries to distance herself from him because of his impertinent letter, but his gentle manners work their spell until she is torn between attraction to him and her belief in his past duplicity.
The unexpected appearance of Mr. Macartney reveals an unexpected streak of jealousy in the seemingly-unflappable Lord Orville. Convinced that Macartney is a rival for Evelina's affections, Lord Orville withdraws. However, Macartney has intended only to repay his financial debt to Evelina.
Lord Orville's genuine affection for Evelina and her assurances that she and Macartney are not involved finally win out over Orville's jealousy, and he secures a meeting between Evelina and Macartney. It appears that all doubts have been resolved between Lord Orville and Evelina, especially when Mrs. Selwyn informs her that she overheard Lord Orville arguing with Sir Clement about the latter's inappropriate attentions to Evelina. Lord Orville proposes, much to Evelina's delight. However, Evelina is distraught at the continuing gulf between herself and her father and the mystery surrounding his false daughter. Finally, Mrs. Selwyn is able to secure a surprise meeting with Sir John. When he sees Evelina, he is horrified and guilt-stricken because she closely resembles her mother, Caroline. Evelina is able to ease his guilt with her repeated gentle pardons and the delivery of a letter written by her mother on her deathbed in which she forgives Sir John for his behavior if he will remove her ignominy (by acknowledging their marriage) and acknowledge Evelina as his legitimate daughter.
Mrs. Clifton, Berry Hill's longtime housekeeper, is able to reveal the second Miss Belmont's parentage. She identifies Polly Green, Evelina's former wetnurse, mother of a girl 6 weeks older than Evelina, as the perpetrator of the fraud. Polly has been passing her own daughter off as that of Sir John and Caroline for the past 18 years, hoping to secure a better future for her. Ultimately, Lord Orville suggests that the unfortunate girl be named co-heiress with Evelina; kindhearted Evelina is delighted.
Finally, Sir Clement Willoughby writes to Evelina, confessing that he had written the insulting letter (she had already suspected this), hoping to separate Evelina and Lord Orville. In Paris, Mr. Macartney is reunited with the false Miss Belmont, his former beloved: separated by Sir John, at first because Macartney was too poor and lowly to marry his purported daughter, and then because his affair with Macartney's mother would have made the sweethearts brother and sister, they are now able to marry because Miss "Belmont"'s true parentage has been revealed. They are married in a joint ceremony alongside Evelina and Lord Orville, who decide to visit Reverend Villars at Berry Hill for their honeymoon trip.