Mr. Herring is head of the Drama Society at Manor Road School. He is also (in his private life) friendly with the parents of Polly Whittacker's friend Nina, and when Nina goes crazy for pop groups that her parents don't approve of, they contrive to get her accepted into the Drama Society. Nina's new aim is suddenly to become a great actress, which she feels shouldn't be hard at all.
Nina persuades Polly (of course) to join too, and Polly becomes a member of the chorus of clowns in the upcoming production of a semi-dance production of Pierrot and Harlequin. Nina (of course) has the starring role of Pierrot. But things happen, and eventually it becomes more Polly's thing than Nina's. And Polly is able to use her gymnastic training to do a good job of playing a sad, sad clown, in a production that is attended by at least one member of her family, as well as by Thomas Lynn. It's a learning experience for various people, for various reasons, all told.