Hastati Squad, the newest batch of freshman cadets at the UNSC's Corbulo Academy of Military Science, pays the price for cadet Thomas Lasky's tactics.

Lasky's team blame him for failing the previous test. Lasky decides it's time to change.

Lasky discovers that he has a severe allergy which would allow him to be medically discharged from service, which leaves him contemplating his future. It is revealed that his brother Cadmon was killed in combat against insurrectionists. Lasky and Silva meet outside the academy grounds and, while discussing his future, share a kiss. In the meantime, cadet Sullivan has decoded classified combat footage which shows UNSC troops, insurrectionists, and Spartans fighting together against a new, unseen foe. An alarm is sounded at the academy, and as the cadets attempt to evacuate via a space elevator, several Covenant vessels appear in the sky, destroying the elevator. An Elite appears out of the smoke as UNSC forces begin to defend the academy.

The Covenant attack overwhelms the UNSC defenses, killing most of the cadets and humans on the planet. As the remainder of Hastati Squad attempts to flee from an Elite under active camouflage, they send a distress signal but are trapped in an armory. Just as the Elite prepares to strike, it is killed by the Master Chief, who escorts Lasky, Orenski, Silva, and Sullivan out of the grounds. Outside they are ambushed by several Jackals and Sullivan is wounded, but the group manages to escape the academy via a Warthog, heading for the forest and evacuation from the planet.

Driving through the dark forest, Chief and the remaining cadets fight off Jackals in the trees. Lasky crashes into one of the aliens on the road, causing the Warthog to stall. While Chief patches up Sullivan, Silva realizes she's been wounded. Chief covers the cadets as they abandon the vehicle in the face of larger aliens called Hunters. Silva hands Lasky dog-tags before dying. The Chief catches up to the cadets and they run from another Hunter. Lasky volunteers to act as a diversion, allowing Chief to kill the last Hunter. The group rendezvous with the Pelican and escape the planet. 32 years later, Lasky, now a Commander on the vessel UNSC Infinity, is seen ordering a jump after listening to a distress beacon sent by Cortana from UNSC Forward Unto Dawn.