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 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.