Mary is a thirteen-year-old schoolgirl. The stage of growth that she's reached, from childhood to adolescence, is illustrated by comparison with the growth of animals. Diagrams show the changes in a girl's body which take place during adolescence, and the reasons for the changes are explained. The reproductive system and the menstrual cycle are demonstrated.

Mary is offered advice on hygiene and general health precautions, including diet and the use of sanitary pads.