Farm Invasion USA is a game about aliens, harvesting corn, and keeping your farm equipment in top shape, and while it sounds a bit muddled (and can be at times) it generally works quite well. The interface is well designed, the game runs smoothly, and there are plenty of upgrades and challenges that will keep you playing for a long time without its' becoming a grind.
When you load up Farm Invasion USA for the first time you get a very basic overview of the plot, and it isn't much help in determining why you are harvesting corn and killing aliens at the same time. The game makes few demands of you, however, so there's really no reason to clarify these details. Move up and down the tracks, harvest your corn to get points, and destroy the aliens with your gun or by running them over, all while avoiding obstacles that will damage your thresher. The effect is a surprisingly agile, fun, endless runner-style app that combines a number of disparate elements into a single addictive experience. There were few or no load time issues, so the play is smooth. And while the interface is cluttered, at times, it is fairly well labeled and the game offers enough depth to keep you playing for a while.
If you are looking for a fun, free diversion, consider downloading Farm Invasion USA. The humor is not always as sharp as the theme and style would dictate, but it still manages to work as a tongue-in-cheek sort of game with plenty of options for how to play and plenty more rewards for successfully gathering corn and fighting off the alien invaders on your farm.