Based on the 1938 Hitchcock thriller of the same name (which I haven't seen), this looked like rather a good whodunnit. The cast if full of young up-comers and old stalwarts, many of whom seem to be doing the rounds in British TV at the moment.
The premise: a beautiful young socialite, Iris Carr, is making her way back to England by train after a Balkans holiday and finds herself befriended by a kind older lady who calls herself Mrs Froy. Disorientated by a fall at the station earlier, Iris drifts off to sleep, only to find on awakening that Mrs Froy has disappeared and nobody else seems to have seen her - in fact they don't believe she existed in the first place. Of course there are only two possible outcomes: the woman isn't real and Iris is barking mad, or she has genuinely disappeared and there's some sort of conspiracy going on.
Unfortunately the final hour dedicated to resolving the mystery is slow-paced, boring and ultimately all a bit predictable. Apart from Sandy McDade and Tuppence Middleton, all the other characters are stereotypes who get to do very little with their screen time. Middleton is superb, tackling Iris's transition from petulant snobbery to concern and brave determination with aplomb, but the plodding script can't keep up with her enthusiasm. It's definitely a Sunday afternoon movie, and one you can watch with Grandma - just don't expect edge-of-your-seat thrills.