It is 2555, more than two years after the Master Chief went missing in action following a decisive conflict on Installation 00 - the massive, extragalactic Forerunner construct known as the Ark - as part of the final chapter in humanity's bloody 30-year struggle against the overwhelming forces of the Covenant. Now, as a tenuous peace exists between the humans and the Elites, a startling scientific discovery is made - and the riddle behind its Forerunner origins could very well seal the fate of the entire galaxy within a matter of weeks.

In order to unravel these dangerous secrets, a heroic, hastily formed coalition of humans and Elites must attempt to overcome their differences as they embark on a covert mission back to the Ark - an astonishing, enigmatic place beyond comprehension from which few have returned and where mortal danger awaits them all....