Star Trek Enterprise is a prequel series that takes place a hundred years after first contact with the Vulcans and a hundred years before Kirk. With no Federation and a new star fleet, Enterprise is the first earth starship. The first to season are spent exploring more like the original series. In the third season however, is different. Aliens in the future are defeated by the Federation in war. Trying to stop this out come they contact through time an alien species called Xindi, telling the Earth was going destroy them in the future to stop the federation from being formed. The Xindi send a test weapon to Earth killing 7 million people. So, the third season has the only Earth ship, Enterprise, trying to track down and stop the Xindi. The fourth season is spent trying to transition after a war and also setting up the franchise being the last season.