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.

Across the galaxy, a woman thought to have died on Reach is actually very much alive. Chief scientist Dr. Catherine Halsey broke every law in the book to create the Spartans, and now she's broken some more to save them. Marooned with Chief Mendez and a Spartan team in a Forerunner slipspace bubble hidden in the destroyed planet Onyx, she finds that the shield world has been guarding an ancient secret – a treasure trove of Forerunner technology that will change everything for the UNSC and mankind.

As Kilo-Five joins the hunt for Halsey, humanity's violent past begins to catch up with all of them as disgruntled colony Venezia has been biding its time to strike at Earth, and its most dangerous terrorist has an old, painful link with both Halsey and Kilo-Five that will test everyone's loyalty to the limit.