Amos Posner has a lovely house in the upscale Hamptons beach community of eastern Long Island. But recent events in Amos's life are preventing him form enjoying it.  His employer, an international trading firm, fired him after making him the scapegoat for some shady business deals.  His wife, a highly successful Manhattan lawyer, has not taken kindly to his job situation, and their marriage is under considerable stress.       Amos is spending most of his time at the beach house, alone, and not at all happy.  So he is highly vulnerable when a beautiful woman approaches him on a bus - the Hampton Jitney - from Manhattan to the Hamptons and persuades him to show her around the area on her day off from her job as a psychiatric resident at a Manhattan hospital.  When Amos reluctantly agrees, he gets far more than an ego boost. He gets a nightmare beyond imagination.  And the cascading events could cost him more than the loss of his job and his wife. They could cost him his life.