Only You Can Save Mankind (1993) is the first novel in the Johnny Maxwell trilogy of children's books by Terry Pratchett.

12-year-old Johnny recieves a pirate edition of the new video game Only You Can Save Mankind from his friend Wobbler. However he hasn't been playing for long when the Evil ScreeWee Empire surrenders to him. After accepting the surrender, he finds himself inside the game in his dreams, where he must deal with the suspicious Gunnery Officer, as well as the understanding Captain, and work out exactly what they're all supposed to do now.

The might all be the result of an over-active imagination, except that the ScreeWee have disapeared altogether from everyone else's copy of the game. With the help of another player, Kirsty, who calls herself "Sigourney" (as in Weaver), Johnny must try to get the ScreeWee home.

The book contrasts the apparent reality of the computer game with Johnny's difficult home life, and the television coverage of the Gulf War which, from Johnny's viewpoint, is just as displaced from reality as the ScreeWee Empire. It plays with the effects of perception; for instance, since Kirsty refuses to see the ScreeWee as anything other than alien monsters who exist to be shot at, they are somewhat less human-like in her presence than when only dealing with Johnny.

ISBN 0552551031