In the year 2552, humans on the planet Madrigal fight for independence from Earth, but a fatal encounter with the Alien Covenant complicates things. Master Chief John 117 and his super-soldier `Spartans' join the fight.

John takes Kwan to an old friend and learns more about the mystery object, which the Covenant and Makee are determined to steal. With the alien threat growing, Dr Halsey has a plan to deal with John's unpredictable behaviour.

John meets his new partner and discovers secrets inside his own memory. Kwan wants to return to Madrigal to continue her people's fight for independence, but Soren has other plans. Makee initiates her plan to retrieve the Madrigal object.

John takes Halsey and Cortana to look for answers from his past. Kwan searches for an army, John's behaviour inspires another Spartan to begin a journey of self-discovery, and Miranda discovers a link between the artefact and something larger.

On the run and out of options, Kwan and Soren have a difference of opinion. With the possibility of finding the mysterious Halo on everyone's minds, John makes a horrible discovery. Also, the war rears its ugly head.

John confronts Dr Halsey and the lies of his past and the new prisoner seems to know John better than he knows himself. Determined to understand more, John turns back to the mystery artefact, which shows him something truly startling.