The universe is under siege. Every fifty thousand years, a race of sentient machines invades our galaxy to harvest all organic life-forms. They are the Reapers. Two people who know the truth are desperately searching for a way to stop the cycle: Navy admiral David Anderson and his partner, Kahlee Sanders. They have uncovered grisly evidence proving that the Reaper threat is real. But in so doing they have exposed the machinations of Cerberus, a secretive paramilitary organization, and its mysterious leader, the Illusive Man - putting David and Kahlee in mortal danger, for Cerberus will stop at nothing to protect its secrets. But along the way, they find an unlikely ally in Gillian Grayson, a young woman with extraordinary powers. Once the subject of horrifying scientific experiments, Gillian is now free - and beginning to master her deadly abilities. But after learning that Cerberus was responsible for the death of her father, Gillian swears vengeance against the group and the Illusive Man - threatening to unravel everything Kahlee and David are fighting for.