Very much classic Hitchcock - After a break-in at her apartment, unsettled journalist with anxiety issues goes on travel reporting cruise aboard a small luxury liner for the ultra-rich. During the trip, she encounters a woman who promptly vanishes, leaving no evidence. Did she really hear a splash? Was she ever really there? It's all a bit "The Lady Vanishes" on a boat instead of a train, but that's no criticism - It's pretty gripping stuff as our heroine investigates while doubting her own memory and mental stability.