[Spoilers below.]

Agatha Christie writes, in At Bertram's Hotel:
There had, of course, been many other hotels on the model of Bertram's. Some still existed, but nearly all had felt the wind of change. They had necessarily to modernize themselves, to cater for a different clientele. Bertram's, too, had had to change, but it had been done so cleverly that it was not at all apparent at the first casual glance.
The plot is this: Bertram's is being used as a front for a coterie of criminals. The employees and even some of the guests are fakes, criminals and actors, paid to create the illusion of the old-fashioned propriety of the bygone Edwardian era. The hotel itself is a sham, a doppleganger.

Trust Agatha Christie to casually pass off psychological horror as a murder mystery...

(link via Doyce.)