It's the summer holidays and Flora has gone off with Dad to the exotic set of his new film and Mum is at home having a much-needed rest with baby Pumpkin. Bluebell, Twig and Jas have been sent to stay with Grandma at Horsehill in the countryside. With Grandma keen that the children get as much fresh air as possible, they are sent off on bikes to go wild swimming and befriend the boys next door. With so much freedom, they can't help but get into trouble, and Grandma doesn't seem to be as capable as looking after them as she should be...