Love Letter in the Sand (1988)

During a month of unusually heavy rain in Tokyo, a mystery assailant has been brutally assaulting and killing people with a metal bat. When a handsome young man with amnesia is found outside an area hospital, the nurse who discovered him begins to suspect that he may in fact be the assailant. But her own strange erotic fixations with him, and the questionable involvement of the doctor tasked with the amnesia victim, lead them all down an increasingly twisted path of deeply-rooted trauma and violence.