Mary Newland is running away with Leslie Perrins, but is having second thoughts. By the time her husband, John Loder, shows up. she's thoroughly disenchanted. Loder assaults Perrins, gets his tickets for the Channel train and demands Mary's letters. When they're disembarking in France, he hasn't got a key for the big trunk he says isn't theirs. When it is opened, Perrins' body is inside. Good thing Peter Haddon, aka Lord Peter Wimsey is there.
It's a well-written short mystery. That's hardly surprising, because Dorothy Sayers wrote an original story for this, her sole original venture into screenwriting. I do have some issues with the casting. Buntner is too old, and too much the trained butler, and Haddon, although he makes a good stab at Wimsey, lacks the character's diffidence. I have long thought that he should have been played by someone like Claude Hulbert, or some other silly-ass British comedian; in one novel, he's said to look like Ralph Lynn.
Casting aside, it's a good quota quicky. the lack of other original stories indicates that Miss Sayers didn't enjoy the experience much. She was moving out of detective fiction anyway, into plays and more academic writing.