from the back cover:
When Ping sees Hanno in the zoo, he is excited, but also unhappy. Hanno is a magnificent African gorilla, big and black and much stronger than a man. But how can this wonderful wild animal live in a cage, behind bars and locked doors?

Then Hanno escapes from the zoo. And a few days later his footprints are seen near Green Knowe, the old house deep in the English countryside where Ping is spending his holiday . . .