Rome, Nora Mariani, an established photographer at an advertising agency, Signal, is haunted by strange phone calls in which a female voice tells her that she is her sister Claudia, who died following a road accident that occurred two years earlier in Germany, in which Nora, who was driving the car, and a German hitchhiker, to whom the two had given a lift, had also been involved; Because of that trauma, Nora was no longer able to drive. The phone calls are increasingly frequent and insistent, so much so that Nora becomes convinced that her sister is still alive.