The novel’s first thirty letters are between Clarissa Harlowe and her best friend, Anna Howe, although some copies of letters to and from other characters are enclosed within these. A dramatic event has just occurred: Clarissa’s brother James has gotten into a fight with the notorious libertine Robert Lovelace, and he lies injured. Anna has heard rumors that the fight was over Clarissa, and she asks her friend to clear up the story. Clarissa explains that Lovelace visited the Harlowes as a suitor to her older sister, Arabella. Arabella admired Lovelace, but he showed a total lack of interest in her and appeared much more attracted to Clarissa. James Harlowe returns from Scotland and learns about the family’s new relationship with Lovelace. James is furious and threatens to disown Clarissa if she ever marries Lovelace.

To explain their history, Clarissa reveals that James and Lovelace had been at college together, where Lovelace had been popular, successful, and something of a bully. James’s pride and bad temper had caused a rift between the two, and Lovelace’s power over his classmates had given him the ability to ruin James’s school experience. Siding with James, the Harlowes (except Clarissa) began to treat Lovelace rudely. James challenges Lovelace, but, as a poor swordsman, loses the fight. Lovelace acts like a gentleman by allowing James to live, and by politely sending inquiries about his recovery, to which the Harlowes respond insultingly.

In the next few weeks the Harlowes become worried that Clarissa will marry the man who is now the family enemy. They forbid her to see Lovelace and propose a new suitor: a rich, ugly man named Roger Solmes. James and Arabella convince Mr. and Mrs. Harlowe to insist on Clarissa’s marrying this man, whom she hates. Clarissa is not allowed to write any letters or leave home until she agrees to marry Solmes. She arranges to carry on a secret correspondence with Anna, and Lovelace convinces Clarissa to also correspond with him by implying that if she does not, he may not be able to contain his anger at the Harlowe’s insults.

Anna writes that Clarissa’s brother and sister are solely motivated by jealousy. From childhood, Clarissa has been admired for her beauty as well as for her virtue and intelligence. When their grandfather died, he left his estate to Clarissa instead of to her older brother and sister. James and especially Arabella have always been in Clarissa’s shadow, and Lovelace’s defeat of James and rejection of Arabella have fueled their resentment.

Anna also suggests that Clarissa is in love with Lovelace. Clarissa is alarmed by this idea and insists that she “would not be in love with him, as it is called, for the world.” Anna mocks Clarissa for her denial, then relates new information she has learned about Lovelace. He is very wild, especially with regard to women. He lives for pleasure but is generous and financially responsible, as well an intelligent and accomplished man of very good family. He is noted for his love of and talent for writing (as is Clarissa).

The Harlowes continue to lobby Clarissa to marry Solmes. His money and his property, which adjoins theirs, will greatly advance the standing of the Harlowes, possibly enabling James to buy a title. Clarissa’s confrontations with her family become increasingly dramatic and she is ever more torn between familial duty and her dislike of Solmes.