The Sensuous Assassin(Qui?)

Marina and her boyfriend have an argument while on a trip in France. While driving with the car among the cliffs, he starts speeding and the car falls into the sea. Marina can jump out of the car, but her boyfriend seems to be drowned. She gets to know his brother and he falls in love with her. But why does she always feel watched? What reasons are behind her strange behaviour anyway? Did she really murder her boyfriend? But is he dead anyway?