The Runaway (S)

On a winter day, Walter Dahlman is getting ready for a job interview; he's an unemployed architect in Hamburg. As he leaves his flat with little time to spare, a boy of about eight stops him in the hall and asks for a ride to school. Walter doesn't know the lad, who says he's Yuri. The boy is persistent; Walter, who has only a bicycle, ends up giving Yuri a lift to school. The boy calls him "Daddy," and it's just the beginning of Walter's trials and tribulations. Who is the boy, where is his mother, and why Walter?