Roger Ebert once said if nothing happens in the first 10 minutes, nothing is going to happen. I believe you can determine during the first 5 minutes of a movie whether it's worthy of viewing. I tried the first few minutes of this half-century-old Agatha Christie-based story, and it locked its clutches on me for a full two hours - even though I already knew the twist ending. Maybe it was Laughton's captivating delivery of dialogue, maybe Dietrich's omni-devilish screen presence, or maybe just the luscious stark black-and-white photography. Another reminder of a bygone era of incredible film-making.
PS #1: It's available here in beautiful HD resolution. I can't make it any easier.
PS #2: Watch for Elsa Lanchester (the Bride of Frankenstein), in a terrifically engaging role as the bossy nurse.
PS #3: Thank you, Maryam, for the recommendation.