For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Clarissa's as happy as a clam before her parents forbid her from talking to her new crush, Lovelace. She's dead-set against marrying gross old Mr. Solmes, but her BFF Anna tries to stop her from doing anything rash. When Lovelace tricks Clarissa into running away with him, her family assumes she's a ruined lady. The stage is set for some serious complication in Act II.
Act II
Clarissa thinks Lovelace is moving her to a more respectable living situation, but her new hangout is actually a brothel. She manages to escape, but Lovelace reels her back in by getting prostitutes to pretend to be his family (y'know, that old trick). Clarissa continues to resist until Lovelace drugs and rapes her while she's unconscious. We're about as far from a happy ending as it's possible to imagine—which is a good place for Act II to end.
Act III
Clarissa's a dying woman, and only Lovelace's pal Belford can help. The prostitutes from Mrs. Sinclair's have Clarissa thrown into jail, but Belford gets her out and finds her a doctor. Lovelace repents and wants to put a ring on it for real, but Clarissa's already bought a coffin. Clarissa dies, everyone repents, and Colonel Morden kills Lovelace in a duel. Bow: tied.