The Covenant seeks Requiem, a place where an ancient Didact, a leader of Forerunner society, resides. This particular Didact is hellbent on destroying the human race, and is the "ancient evil that threatens the fate of the entire universe," of the game's official description.
Cortana has a problem and Master Chief enlists the help of Dr. Halsey to help solve it.
The script, pictured at 3:55 in the A Hero Awakens trailer, reveals a climactic cutscene with Cortana and Master Chief battling Didact. Cortana at one point makes multiple projections of herself -- "all of the Cortanas emerging from the Bridge." Didact, who is more than 13 feet tall, grabs Master Chief and drops him as Cortana manages to subdue the Forerunner. The script cuts off after Master Chief falls, lands on an edge and is left "hanging idle."