Jimmy Stoker's dreams have been full of tigers since he started his Saturday job at the zoo. Boris, the great Siberian tiger, has died and rumours abound about what's going to happen to him now. Jimmy hears that someone wants to steal Boris's bones, because tiger bones reputedly have healing powers and fetch a lot of money on the black market. So when he sees the tiger being smuggled out of the zoo, he fears the worst. With the help of his schoolmate Billy and his old friend Major Gregory, Jimmy is determined to investigate.