Shandy is only a little suspicious when the invitation arrives: it’s handwritten on expensive paper, no return address, and it is enclosed with a plane ticket and details of a fabulous cruise. It isn’t junk mail, so that more or less passes rigour.
Once she’s actually on the boat, however, it becomes clear very quickly that someone is going to be murdered.
There’s only a handful of them, in their fine clothes, with their strange stories, and they’re completely oblivious. There are at least five ominous statements made at dinner. She brings her camera everywhere. She’s sure she’ll need it.