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.
The diamond is stolen!
Franklin Blake figures out that he took it…but doesn't remember doing it.
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.