A popular actress/singer's body is found on the beach. At first it is believed to be an accidental drowning, but soon it becomes apparent that this is murder. The novel begins well, and there are some interesting characters, but then it begins to go downhill. Instead of vintage Tey, we seem to have an Agatha Christie plot, and not the best Christie.
There are red herrings galore, a false arrest, 2 escapes, supernatural overtones, and then an unexpected, unprepared for, and totally implausible ending. That the guilty one would have and could have perpetrated the crime in the manner indicated is just beyond belief. The author even has to make her turn out to be insane to create a strong enough motive--no, not nearly strong enough actually.
If you want to read excellent Josephine Tey, try BRAT FARRAR or THE FRANCHISE AFFAIR. This one just isn't up to her later standards.