Just hours following their climactic battle on the Forerunner planet Genesis, the Spartans of Blue Team and Fireteam Osiris find themselves running for their lives from the malevolent machinations of the now-renegade artificial intelligence Cortana. But even as they attempt to stay one step ahead, new trouble seems to find Spartan Edward Buck no matter where he turns.

A secret mission planned by the Office of Naval Intelligence could possibly help turn the tide, and has Buck reluctantly agreeing to reform his old team, Alpha-Nine. But if the band is really getting back together for this one, that means everybody-including the Spartan who Buck never wants to see again, the one who committed the ultimate betrayal of trust...