The Covenant has collapsed after a long, brutal war that saw billions slaughtered on Earth and her colonies. For the first time in decades, however, peace finally seems possible. But though the fighting's stopped, the war is far from over: it's just gone underground. The UNSC's feared and secretive Office of Naval Intelligence recruits Kilo-Five, a team of ODSTs, a Spartan, and a diabolical AI to accelerate the Sangheili insurrection. Meanwhile, the Arbiter, the defector turned leader of a broken Covenant, struggles to stave off civil war among his divided people.