A lesson on human biology is announced, with a letter to be taken home for permission to be sought. Can H3 be sensible about the subject? What will Anita Unsworth's Gran say? Matthew Cartwright would rather not do the lesson, and takes some stick from older sister Pamela. Pogo tries to make a few bob out of the biology textbook's "disgusting pictures". Claire's mother speaks to Mrs. McClusky concerned that her daughter is too innocent for the forthcoming biology lesson. How did Matthew Cartwright lose his trousers? It's all chaotic until Mrs McClusky asks the form on their opinion, and the conclusion is they do want sex education. Pogo and Gripper are interrogated by McClusky about their bad behaviour.