The mission starts as John-117 takes out an Elite from the Covenant Remnant. The boarded Lich’s course was too low and due to her rampancy, Cortana ignored the Chief’s repeated attempts to get her to correct their course, resulting in a collision with the Ivanoff Research Station.
The Chief then fights through Covenant forces who were boarding the station to find his way to Dr Tillson. The Covenant were here to remove the Composer for the Didact.
The Chief met up with Doctor Sandra Tillson to inform her that the station’s personnel must be evacuated, and that the Composer had to be destroyed using a nuke to prevent the Didact from claiming it. The Chief then attempts to hold off Covenant forces so that they could destroy the Composer. However, the Didact removes the Composer using a tractor beam from his ship. The Chief makes his way to Dr Tillson. Unexpectedly, the Didact uses the Composer on the station, turning all the humans on it, including Dr Tillson to ash. Fortunately, Master Chief was unaffected, thanks to the Librarian’s genesong, given in the level Reclaimer, which made him immune to The Composer’s effects. The Chief and Cortana then obtain a Broadsword from the station’s hangar bay and equip it with a HAVOK Tactical Nuclear Weapon to pursue the Didact and destroy his ship before it reached Earth. (Source)