It's been three months since the colony world of Meridian was invaded by the alien theocratic alliance known as the Covenant. Under the close watch of the military, teenagers Evie, Dorian, Saskia, and Victor have been put into an accelerated training program with ONI, the Office of Naval Intelligence. And to the quartet's surprise, ONI has a mission for them: return to their hometown on Meridian and monitor the Covenant's efforts to retrieve an ancient Forerunner artifact of untold power.

But what seems like a simple job quickly spirals out of their control. With the artifact at risk of falling into Covenant hands, the stakes are raised, and ONI tasks the teens and their team of militia fighters with extracting the artifact for study. After a series of missteps with command, the group must take matters into their own hand, journeying far behind enemy lines and into the heart of the war zones on a ravaged Meridian.