Poor Jane Helier, she really is treated as just a ditzy pretty face, isn't she? I'd complain about it, but Christie's had other beautiful women who aren't so dim, or are even shrewd, so I at least I guess it's just a character quirk, not a stereotype.
And man, I always feel bad for people who ARE trying to tell stories about (or get help for) a real life actual friend, because you know no one's going to believe it. And their specific reasons for being suspicious here are especially cruel (ie, she's too selfish and/or dumb to think about anyone else.) Still though, I can't help but be amused at by how it's handled. (Sir Henry wondering how long before she starts using "I," etc...)
Miss Bantry's very careless I think about her jewels - not so much leaving them unlocked, but just blabbing out to anyone interested where she keeps them!
Anyhow, the bad thing about this case is that we don't get to see how Miss Marple figured it out really, or that it hasn't happened yet, or... well, anything. We do get the result with Jane Helier's confession to Mrs. Bantry, but that's about it. Boo.
And it seems rather out of character for Miss (Mrs.?) Helier to come up with what seems to me a fairly clever plan like this, and not give herself away, when she couldn't even give away that it wasn't her that it was (would be?) happening to. Unless she's an even better actress than people give her credit for, and she's really not nearly as dim as she sees - but why would she pretend to be quite so vapid, all the time?
(And I feel bad putting down even a fictional woman like this by calling her dim and vapid all the time, but I don't know what else to call her, because dude!)
Anyhow, in some ways this is one of the most unsatisfying stories to me, because as mentioned we don't get any of Miss Marple's thoughts on the whys or wherefores of the solution, and because it's all "could-be's" rather than actual (well, in terms of the fictional story!) occurrences.
Any other thoughts?