The best thing about Agatha Christie is that even when you think you know where it is going, she is a step ahead of you. You get to sit there thinking you have it figured out, only to find out that you didn't know the half of it.
Charles Laughton, Tyrone Power, Marlene Dietrich, Elsa Lancaster, John Williams and many others make a spectacular cast, and there is not a bad performance among them.
If you love courtroom dramas, this is the granddaddy of them all--two years before Anatomy of a Murder, three years before Inherit the Wind, five years before To Kill A Mockingbird, and unlike 12 Angry Men this one actually has dramatic scenes that take place IN the courtroom.
And please, if you are watching it with someone seeing it for the first time, let them enjoy it and don't give away the ending!