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
The diamond is stolen!
Act II
Franklin Blake figures out that he took it…but doesn't remember doing it.
Act III
Mystery solved: Franklin took it while drugged with opium, but the Godfrey Ablewhite is the one who really stole it. But then Godfrey gets murdered.