Johnny Maxwell is a perfectly average twelve year old (with, perhaps, an embarrassment of interesting friends available to serve as supporting characters). His parents are splitting up and he spends a lot of time playing bootlegged video games. He's quite surprised when the bad guys - aliens - in his latest game surrender. It's just that they surrender so aggressively. They refuse to shoot back at him or other human players and they start fleeing computer screens all over the country.
It turns out it's not so much fun to fight people who have an infinite number of lives.
Pratchett layers on some oblique (OK, not actually so oblique) political commentary, as this is set during the first Gulf War. Johnny's buddies are hilarious, as is the girl geek he eventually teams up with. A very competent YA book.