Clarissa tells the story of a virtuous, beautiful young woman who is brought to tragedy by the wickedness of her world. The eighteen-year-old Clarissa Harlowe is universally loved and admired, considered an exemplary woman by everyone around her. The Harlowes are an up-and-coming family, possessing great wealth but little status. The other members of the family are avaricious and eager to improve their standing in the world, and Clarissa becomes the victim of their greed. The trouble starts when Richard Lovelace, a dashing libertine, comes to pay court to Clarissa’s sister, Arabella, but is attracted by Clarissa instead. Arabella’s jealousy combines with the resentment of their brother, James, who holds a grudge against Lovelace from college days, and sets the family against him.
A duel between the two, in which Lovelace wounds James but spares his life, crystallizes their hatred. The family becomes suspicious of Clarissa, forbids her from corresponding with Lovelace, and commands her to marry a horrible rich man named Roger Solmes. Clarissa refuses to consider marrying Solmes and carries on a clandestine correspondence with Lovelace. She also continues to secretly correspond with her best friend, Anna Howe. As she continues to resist marriage to Solmes, Clarissa is increasingly confined, until she is barely able to leave her room. Finally Lovelace takes advantage of Clarissa’s fear of a forced marriage by tricking her into running away with him.
Once Clarissa has run away, she is in Lovelace’s power. Her reputation is ruined and her family refuses to forgive her. Lovelace is an adept manipulator, enjoying the “contrivances” he invents to keep Clarissa in his web. He is in love with her, but he hates the idea of marriage, so his goal is to force her into “cohabitation,” rather than marriage. Clarissa is innocent and virtuous and does not see through Lovelace’s tricks. Furthermore, she refuses to compromise any of her strict tenets of behavior, even to save herself. Lovelace repeatedly tests Clarissa’s virtue as a means of testing the character of the entire sex: if Clarissa is truly an exemplary woman, she will withstand his contrivances and remain a model of goodness. His intention, however, is to force Clarissa to compromise her strict morals, sully her reputation, and gain full control over her. Without suspecting that she is playing into his hands, she goes with him to London, where he secures lodgings at Mrs. Sinclair’s house. Clarissa is unaware that this is a brothel and the women she meets there are whores. Having been involved with (and ruined by) Lovelace in the past, these women are jealous of Clarissa and encourage Lovelace to rape her.
At the same time, Clarissa’s virtue has a powerful effect on Lovelace and sometimes sways him away from his bad intentions. After several battles between his wicked heart and his protesting conscience, Lovelace’s joy in intrigue and the whores’ instigations seal Clarissa’s doom. Finally suspecting Lovelace’s vileness, Clarissa escapes, but Lovelace finds her and tricks her back to Mrs. Sinclair’s brothel. There, Mrs. Sinclair drugs Clarissa and Lovelace rapes her while she is unconscious. When she awakes, Clarissa goes temporarily mad, and Lovelace regrets his action. The rape has failed to put Clarissa fully in his power because she has never compromised her virtue. He begins to talk with more seriousness about marrying her, but also thinks he will try to rape her again and see if he can get her consent, thus abandoning her principles. Clarissa, sensing the danger, runs away, this time successfully.
Once Clarissa has been raped, she stops eating and no longer worries about worldly problems like reputation. She continues to seek reconciliation with her family, but they remain adamant. One of Lovelace’s plots gone wrong allows him to accidentally discover Clarissa’s location, but at the same time it damages her health and cements her conviction of his wickedness. Lovelace’s friend Belford becomes Clarissa’s protector, keeping Lovelace away but mediating between him and Clarissa. Lovelace is now truly determined to marry Clarissa, but she prefers the idea of death to that of marrying such a criminal. Her health steadily worsens, and she begins to prepare for death.
With remarkable equanimity, Clarissa makes her will, appoints Belford her executor, puts her affairs in order, and even orders a coffin. She finally dies, expressing forgiveness for everyone in her life and joyful anticipation of heaven. The Harlowes finally see how wrong their treatment of Clarissa has been. Mr. and Mrs. Harlowe die soon after, and James and Arabella marry badly and are miserable for the rest of their lives. Lovelace fails to reform and is killed by Clarissa’s cousin Morden in a duel. Anna, Hickman, Belford, and the other good characters are rewarded with happy marriages. Belford takes on the project of collecting the letters that tell Clarissa’s story so that it can be an example to protect other women from similar fates.