In the year 2552, humans on the planet Madrigal have been fighting 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 he 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 for her. Makee initiates her plan to retrieve the Madrigal object, with consequences.

Guided by his visions, John takes Halsey and Cortana back to where it all began, looking for answers from his past. With Soren along for security, Kwan goes in search of an army. Meanwhile, John's behaviour inspires another Spartan to begin a journey of self-discovery.

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. And the war rears its ugly head.

Survivors from battle return to Reach. John confronts Dr. Halsey and the lies of his past. 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 amazing.

Kwan's journey to uncover her family's "true purpose" takes her deep into the deserts of Madrigal. Soren obsesses over unfinished business.

John and Makee come to terms with their shared vision. Halsey has one last chance to save her mission, and herself. As things come to a head, everyone must choose a side.

Beaten, battered, and betrayed, John 117 leads the Spartans on a suicide mission to find the Halo and save humanity. But at what price?