After the death of his Russian mother, nine-year-old orphan Peter lives with Professor Hartmann and his daughter Helga, who cares for him like a mother. Peter is very musical. He especially loves the melancholy songs of the Don Cossack Choir, which remind him of his mother. Because the boy has suffered from a severe heart defect since birth, his doctor, Dr. Stark, has forbidden him any excitement, including music. When one day the Don Cossack Choir's tour bus breaks down near Peter's house with ...