Knife in the Water (1962)

While heading to a lake for a leisurely afternoon, married couple Andrzej and Krystyna nearly hit a young hitchhiker. They decide to bring him along on their boat trip, but Andrzej quickly begins to psychologically taunt the newcomer. In turn, the hitchhiker flirtatiously engages with Krystyna. When an accident causes the hitchhiker to fall overboard, Andrzej's frantic reaction leads to unforeseen and dramatic developments, intensifying the tension among the trio.