Britain is at war with Nazi Germany. Eleven-year-old Eric spends his days at the place that makes him most happy: London Zoo. And there's one animal in particular he loves: Gertrude the gorilla. With bombs falling all over London, Eric must rescue Gertrude. Together with his Uncle Sid, a keeper at the zoo, the three go on the run. But while hiding out at the seaside they uncover a top-secret Nazi plot.