Robert Lovelace, a wealthy "libertine" and heir to a substantial estate, begins to court Arabella, Clarissa's older sister. However, she rejects him because she feels slighted by his more ardent interest in her parents' approval than in her. Lovelace quickly moves on from Arabella to Clarissa, much to the displeasure of Arabella and their brother James. Clarissa insists that she dislikes Lovelace, but Arabella grows jealous of Lovelace's interest in the younger girl. James, also, dislikes Lovelace greatly because of a duel the two had once fought. These feelings combine with resentment that their grandfather had left Clarissa a piece of land and lead the siblings to be aggressive to Clarissa. The entire Harlowe family is in favour of her marrying Roger Solmes; however, Clarissa finds Solmes to be unpleasant company and does not wish to marry him, either. This makes her family suspicious of her supposed dislike of Lovelace and they begin to disbelieve her.
The Harlowes begin restricting Clarissa's contact with the outside world by forbidding her to see Lovelace. Eventually they forbid her to either leave her room or send letters to her friend, Anna Howe, until Clarissa apologises and agrees to marry Solmes. Trapped and desperate to regain her freedom, Clarissa continues to communicate with Anna secretly and begins a correspondence with Lovelace while trying to convince her parents not to force her to marry Solmes. Neither Clarissa nor her parents will concede. They see her protests as stubborn disobedience and communication between parents and daughter breaks down. Lovelace convinces Clarissa to elope with him to avoid the conflict with her parents. Joseph Leman, a servant of the Harlowe family, shouts and makes noise so it may seem like the family has awoken and discovered that Clarissa and Lovelace are about to run away.
Frightened of the possible aftermath, Clarissa leaves with Lovelace but becomes his prisoner for many months. Her family now will not listen to or forgive Clarissa because of this perceived betrayal, despite her continued attempts to reconcile with them. She is kept at many lodgings, including unknowingly a brothel, where the women are disguised as high-class ladies by Lovelace so as to deceive Clarissa. Despite all of this, she continues to refuse Lovelace, longing to live by herself in peace.
The villain, Robert Lovelace, abducting Clarissa HarloweLovelace is too cynical to believe that virtuous women exist and he is desperately trying to seduce Clarissa, despite declaring that he loves her. Although he puts her under increasing pressure to submit, Clarissa does not waver. Under the pretense of saving her from a fire, Lovelace at last gains entry to Clarissa's bedroom but she thwarts his attempted assault with vigorous resistance. She promises, under threat of rape, to forgive and marry him. However, she considers this promise made under duress as void; soon after she makes her first successful escape from Lovelace, concealing herself in lodgings in Hampstead.
Enraged by Clarissa's flight, Lovelace vows to seek revenge. He hunts her down to the lodgings where she is hiding and rents all the rooms around her, effectively trapping her. He hires people to impersonate his own respectable family members to gain her trust. During this time he intercepts a letter to Clarissa from Anna Howe warning her of true extent of his deception and roguery. He commits forgery to put an end to the communication between them.
Eventually, he persuades Clarissa to accompany his imposter-relatives out in a carriage and thus carries her back to the disguised brothel. There, with the assistance of the prostitutes and brothel madam, he first drugs and then rapes her.
After the rape, Clarissa suffers a loss of sanity for several days, presumably brought on by her extreme distress as well as the dose of opiates administered to her. (This temporary insanity is represented in her "mad letters" by the use of scattered typography.)[3]
When Clarissa recovers her senses Lovelace soon realises that he has failed to "subdue" or corrupt her; instead she is utterly repulsed by him, repeatedly refusing his offers of marriage despite her precarious situation as a fallen woman. She accuses him of deceiving and unlawfully detaining her and insists that he set her free. He continues to claim that the impersonators really were his family members and that his crime was simply one of desperate passion. He tries to convince her to marry him, alternating between threats and professions of love. She steadfastly resists and attempts several more escapes.
Lovelace is forced to concede that Clarissa's virtue remains untarnished, but he begins to convince himself that the "trial" was not properly conducted. Since Clarissa was drugged at the time, she could neither consent nor refuse. He decides to orchestrate a second rape, but without drugs. Pretending to be angered by the discovery that she has bribed a servant to help her escape, Lovelace begins to menace Clarissa, intending to escalate the confrontation to physical violence but she threatens to kill herself with a pen-knife should he proceed. Utterly confounded by her righteous indignation and terrified by her willingness to die for her virtue, Lovelace retreats.
More intent than ever to make Clarissa his wife, Lovelace is called away to attend his dying uncle from whom he is expecting to inherit an Earldom. He orders the prostitutes to keep Clarissa confined but well-treated until he returns. Clarissa escapes; however, the brothel madam is able to have her jailed for a few days for unpaid bills, and Clarissa finds sanctuary with a shopkeeper and his wife when she is released. Corresponding with Lovelace's real family, she discovers for herself the true extent of his deception. She lives in constant fear of being found by him again, as he continues to send her marriage offers through his disreputable friend, John Belford, as well as through his own family members. Clarissa is determined not to accept. She becomes dangerously ill from the stress, rarely eating, convinced that she will die soon.
Her illness progresses. She and Belford become correspondents. She appoints him executor of her will as she puts all of her affairs in order to the alarm of the people around her. Belford is amazed at the way Clarissa handles her approaching death and laments what Lovelace has done. In one of the many letters sent to Lovelace, he writes, "if the divine Clarissa asks me to slit thy throat, Lovelace, I shall do it in an instance".
Eventually, surrounded by strangers and her cousin, Col. Morden, Clarissa dies in the full consciousness of her virtue and trusting in a better life after death. Belford manages Clarissa's will and ensures that all her articles and money go into the hands of the individuals she desires should receive them.
Lovelace departs for Europe and continues to correspond with Belford. Lovelace learns that Col. Morden has suggested he might seek Lovelace and demand satisfaction on behalf of his cousin. He responds that he is not able to accept threats against himself and arranges an encounter with Col. Morden. They meet in Munich and arrange a duel. Morden is slightly injured in the duel, but Lovelace dies of his injuries the following day. Before dying he says "let this expiate!"
Clarissa's relatives finally realise they have been wrong but it comes too late. They discover Clarissa has already died. The story ends with an account of the fate of the other characters.
