Another "Fox Film Noir" oldie from 1953. Again, a definition of "noir" out of my comfort zone. But taken on its own, it's all right.
Jeanne Crain and Carl Betz (Mr. Donna Reed himself) play newlyweds Ruth and John, boarding the SS Monrovia for a honeymoon trip to Europe, after a whirlwind romance and quickie marriage. But no sooner do the loving couple set up in their stateroom than John goes missing. Worse, nobody else on the ship seems to have noticed him, and any documentation that might have proven his existence is absent. So Ruth's increasingly frantic insistence that his disappearance be investigated is met with skepticism. The crew don't quite make the "she's crazy" finger gesture behind her back, but it's close. Michael Rennie plays the ship's doctor, and is slightly less skeptical than anyone else of Ruth's story.
Clue: a maid who we (the audience) know saw Ruth and John together is now lying about it. And Ruth gets phone calls from John telling her that she's in terrible danger and to trust nobody. So what's going on?
Downside: yes, there's a plot going on, but (without giving too much away) even at the end, it's unclear exactly how the scheme was supposed to work. I'm not alone: searching around the Internets finds people similarly confused about it. I can think of some possible explanations, but the movie should have explained itself better.