It is the year 2287, one of the first colony ships sent out by Earth arrives at the planet Maria, in the Epsilon Indi Star System. During its landing sequence, something appears in front of the massive ship that causes it to lose control and crash into the planet. Some of the colonists survive in lifeboats, but what they find is an unforgiving land filled with creatures ready to outlast the visitors from Earth. Now the survivors must explore, gather, build, create, fight, and ultimately, conquer the land.
Here's a demonstration of some tower defense game play in Planet Explorers. By dropping something called a monster beacon, the player can initiate a tower defense mini game at any time, anywhere. This is early though, so you'll see a bunch of bugs in this vid.