October 2559. It has been a year since the renegade artificial intelligence Cortana issued a galaxy-wide ultimatum, subjecting many worlds to martial law under the indomitable grip of her Forerunner weapons. Outside her view, the members of Blue Team--John-117, the Master Chief; Fred-104; Kelly-087; and Linda-058--are assigned from the UNSC Infinity to make a covert insertion onto the ravaged planet Reach.

Their former home and training ground--and the site of humanity's most cataclysmic military defeat near the end of the Covenant War--Reach still hides myriad secrets after all these years. Blue Team's mission is to penetrate the rubble-filled depths of CASTLE Base and recover top-secret assets locked away in Dr. Catherine Halsey's abandoned laboratory--assets which may prove to be humanity's last hope against Cortana.

But Reach has been invaded by a powerful and ruthless alien faction, who have their own reasons for being there. Establishing themselves as a vicious occupying force on the devastated planet, this enemy will soon transform Blue Team's simple retrieval operation into a full-blown crisis. And with the fate of the galaxy hanging in the balance, mission failure is not an option...