Four years after the events of Halo 3, UNSC Forward Unto Dawn drifts towards Requiem from the destroyed ring. Cortana wakes Master Chief from cryonic sleep shortly before forces from a rogue post-war Covenant faction boards the vessel. The remnants of Forward Unto Dawn are caught in Requiem's gravity well and pass through an opening to crash-land on its interior.
As Chief and Cortana explore Requiem, fighting hostile Covenant and mechanical warriors called "Promethean Knights", Cortana malfunctions. She reveals that she is experiencing "rampancy", a declining mental state where AIs past their operating expectancies "think" themselves to death. Chief promises to get Cortana to Earth, believing that Cortana's creator, Doctor Catherine Halsey, can fix Cortana's condition.
Chief and Cortana pick up garbled transmissions from a human ship, UNSC Infinity, who have picked up Dawn's distress call. Cortana attempts to warn Infinity away from Requiem's gravity well and directs the Chief to deactivate what she believes are communications jammers.
Instead, the Chief unwittingly releases the Didact, an ancient Forerunner warrior, from imprisonment. The Didact takes control of the Prometheans and Covenant, and attacks Infinity after it is dragged into Requiem.