what halo's about

The story is about the earths army "Marines" but there space station gets invaded by the covernat and they crash-land on an artificial ring-world called "Halo", a navy captain, his surviving marines, and a chemically-and-surgically-enhanced supersoldier named John-117 "Master Chief" must find out what the Covenant, the genocidal alien race they are at war with, are looking for on the ring. However, Halo holds more secrets than either side imagined.