The opening text tells of how an alien force has harvested the population of Earth over a 100 year time period. Every 25 years they would land and imprison whole areas in shields that prevented anyone from leaving and any outside help getting in. The aliens would harvest the population of the area within a time limit of one day before moving on to another area. After repeating this several times they would teleport back to their homeworld, an artificially created comet, before returning exactly 25 years later as it has completed its orbit around the nearby star systems back to earth. ..