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.