It is 2553, and the three-decade long Covenant War that defined a generation has suddenly drawn to a close. Yet in the remotest parts of human space, tensions remain that threaten to overflow into another full-scale conflict. Beneath the surface of the planet Gao lies a vast cavern system renowned for its therapeutic effects and rumored miraculous cures. But now Gao natives are turning up brutally murdered down there - violent acts that happen to coincide with the recent arrival of a UNSC research battalion.

It is 2555, more than two years after the Master Chief went missing in action following a decisive conflict on Installation 00 - the massive, extragalactic Forerunner construct known as the Ark - as part of the final chapter in humanity's bloody 30-year struggle against the overwhelming forces of the Covenant.

The military-grade artificial intelligence known as Iona has only one week to live. After that the UNSC will legally terminate her seven-year existence in order to stave off the threat of the data corruption phenomenon known as "rampancy", a condition that will eventually take hold of her functionality and persona, endangering all those around her. In a last-ditch effort to save herself, Iona has successfully launched an unprecedented legal appeal against her own death sentence.

It's 2535. It is the first, desperate days of the Human-Covenant War, and the United Nations Space Command has enacted “the Cole Protocol” to safeguard Earth and its inner colonies from discovery by its merciless alien foe. Many are called upon to rid the galaxy of lingering navigation data that could potentially reveal the location of Earth and ensure the destruction of humanity. 

It's 2553. Welcome to humanity’s new war: silent, high-stakes, and unseen. This is a life-or-death mission for the Office of Naval Intelligence’s black-ops team, Kilo-Five, which is tasked with preventing the ruthless Sangheili, once the military leaders of the alien alliance known as the Covenant, from regrouping and threatening humankind again. What began as a routine dirty-tricks operation - keeping the Sangheili occupied with their own insurrection - turns into a desperate bid to extract one member of Kilo-Five from the seething heart of a brutal civil war.