"The world has been overrun by Zombies! Use the aircraft to destroy the mother hive deep in zombie territory. Get there in time... Upgrade your aircraft, fly through different areas- and kill as many zombies as you can!"
Your plane is constantly flying to the right and you just have to press left and right to make your plane fly higher or lower. It you get a zombie in your sights it will automatically shoot at it. The same goes for exploding barrels, which have a nice devastating effect to those zombies around them. The aim is to get to the end of each level and progress across the map. It's just the hundreds of zombies standing in your way.
The plane at the start is pretty weak. It doesn't hold much fuel, the gun is slow and the engine not very fast. But, as with all games in this genre, the further you get and the more zombies you toast, the more money you earn. And that money can be spent on powering-up your plane. From increasing the fuel capacity to improving the hardiness of the body.
So quite simply - waste zombies and improve your plane, which in turn makes you even more effective at wasting zombies :) See how far across the map you can progress and in how short a period of time, as each death costs you a day. It's great fun, the graphics are cartoony and funny and the power-up mechanic is utterly addictive.