Emma, a romantic comedy set in England in the early 1800s, concerns the beautiful, clever, and rich Emma Woodhouse. Emma lives with her hypochondriac father and was raised primarily by a governess, Miss Taylor (Emma's mother died when she was five). Miss Taylor has recently married Mr. Weston, a widower with a grown son, and Emma misses her terribly. The other key person in Emma's life is Mr. Knightley, an English gentleman and close family friend who lives on a nearby estate. Both the Woodhouses and Mr. Knightley are landed gentry, but Mr. Knightley has mostly land, while Mr. Woodhouse has mostly money. Mr. Woodhouse's other daughter, Isabella, is married to Mr. Knightley's younger brother, John.
In her effort to fill the void left by the new Mrs. Weston, Emma befriends the young and beautiful Harriet Smith and resolves to improve her new friend's social standing. Harriet is being courted by a young, respectable farmer, Mr. Robert Martin, but Emma thinks Harriet can marry up. Therefore, Emma begins interfering, first by convincing Harriet to turn down a proposal of marriage from Mr. Martin, and then by introducing her to the young and handsome vicar Mr. Elton. Mr. Knightley is angry when he finds out, as he is a friend to Mr. Martin and thinks the match is a good one. Soon, a misunderstanding develops, in which Mr. Elton thinks he is courting Emma and Emma thinks he is courting Harriet. When Mr. Elton proposes to Emma, she is shocked. She explains that she thought he was in love with Harriet. In fact, Mr. Elton wants to raise his own social standing and was hoping to marry an heiress. After they part company acrimoniously, Mr. Elton leaves town. Emma is somewhat chastised by this fiasco and vows to never attempt matchmaking again.
After Mr. Elton leaves, Frank Churchill comes to town. Although Frank is the son of Mr. Weston and his late wife, he was raised by Mr. and Mrs. Churchill, his rich aunt and uncle on his mother's side. Mrs. Churchill is very possessive and doesn't like to share Frank with anybody. Frank has come to visit his father because he is secretly engaged to Jane Fairfax, the niece of a local spinster, Miss Bates. Jane has recently come home to spend time with her family before going to work as a governess. Frank keeps the engagement a secret because he knows his aunt will never allow him to marry a penniless girl.
Mr. and Mrs. Weston bring Frank to Hartfield, the Woodhouse estate, and a friendship develops between Emma and Frank. Soon, Frank begins flirting with Emma as a way to cover up his real love interest. Emma is flattered by the attention from the handsome and fun-loving stepson of her friend and former governess, Mrs. Weston. The two of them plan a ball, but before it can take place, Frank is called back to his aunt's estate. Emma feels let down, and she thinks she might be a little in love with Frank.
Mr. Knightley dislikes Frank, partly because Frank has been courting Emma, whom Mr. Knightley is secretly in love with, but mostly because he sees through the charming younger man. He begins watching Frank, and at some point he realizes Frank is playing a double game with Jane and Emma.
Mr. Elton returns to Highbury with a rich and obnoxious wife. When Frank comes back a second time, the ball is held, and Emma notices how handsome Mr. Knightley is. At one point during the ball, Harriet needs a partner, and Mr. Elton refuses to dance with her, mostly to hurt Emma. Mr. Knightley steps in to dance with Harriet and saves her from humiliation. Emma then admits to Mr. Knightley how wrong she had been to try to match Harriet with Mr. Elton.
Not long after the ball, Frank helps Harriet escape from a rowdy band of gypsy children demanding money. Harriet tells Emma that she is finally over Mr. Elton but that she is now in love with someone else above her rank. Emma encourages her while telling her to be cautious; she thinks Harriet loves Frank. By this time, Emma knows she is not in love with Frank and also realizes that he doesn't care for her, so why not pair him with her friend? However, she takes no action, as she has promised not to meddle.
The Highbury set have two more outings, the second one a picnic to Box Hill, home of Mrs. Elton's sister. Frank is in a bad mood at the picnic, and he begins flirting shamelessly with Emma because he is quarreling with Jane. Emma is also out of sorts, and as a result she makes a cutting remark to the defenseless spinster Miss Bates. Mr. Knightley rebukes Emma afterward with stern words, saying what she did was cruel and did not befit her status as a lady. Emma is crushed by the reprimand and, as a result, begins to pay more attention to the feelings and needs of others. After the Box Hill picnic, Frank abruptly returns to his Churchill relatives.
Not long after, Emma and the others at Highbury learn that Frank's aunt, Mrs. Churchill, has died. Following closely upon the heels of that news is word of Frank and Jane's engagement; Frank's uncle has agreed to the marriage. Mrs. Weston speaks to Emma, worried that Frank has been leading her on, but Emma explains that she had no feelings for him. In the meantime, Emma is feeling terrible remorse about misleading Harriet for a second time in affairs of the heart. Harriet, however, confesses it is Mr. Knightley, not Frank, that she loves. Emma is both astonished and crushed. She suddenly realizes that "Mr. Knightley must marry no one but herself!" She cannot bear the thought of not being first in his affections. She says nothing to Harriet and hopes her young friend is wrong in thinking that her affection is returned.
Mr. Knightley, who has been visiting his brother and Isabella in London, rushes back to Highbury when he hears about the engagement. He wants to comfort Emma, thinking she is in love with Frank. When he finds her in the garden, Emma reveals to him that she has never been in love with Frank. Mr. Knightley then tells her that he loves her, and she accepts a proposal of marriage. Now Emma has to tell Harriet yet again that the man Harriet loves has proposed to her. Harriet goes to London to stay with Isabella for a time. In the meantime, Mr. Knightley contrives to send Mr. Martin to London on business, where he reunites with Harriet. Soon they announce their engagement, and Emma is overjoyed that her friend has found happiness, despite Emma's presumptuous interference. Mr. Knightley decides that he will move into Hartfield so that Mr. Woodhouse's life is not disrupted by Emma's marriage. By the end of the novel, Mr. Martin marries Harriet, Mr. Knightley marries Emma, and Frank is to marry Jane as soon as the mourning period for his aunt is over.