A woman travels in a compartment of a train and a young man sits next to her with whom, almost immediately, sparks strike: once they arrive at their destination the boy realizes that the young lady has forgotten a book and so follows her to to return it. When the two are alone at home, without having even introduced themselves, they soon end up in each other's arms and spend an intense night of love. At dawn the man leaves. Also at dawn that same morning, a man is killed in his garage and it is soon discovered that he is the brother of the young man on the train.