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
Esther comes to London with her past shrouded in mystery, but her future so bright she's gotta wear shades.
Act II
Tulkinghorn, Bucket, Guppy, and Smallweed start investigating the connections between Esther and Lady Dedlock.
Act III
Suicidal, Lady Dedlock runs off, knowing that her secret is about to be revealed. Esther and Bucket set off on an ultimately fruitless search to find her before she kills herself.