The year is 2557 AD. Four years, seven months and ten days have passed since the activation of Installation 04B and the annihilation of the Flood. While humanity and the Covenant have struck a truce for the time being, and started to rebuild their civilizations, Master Chief Petty Officer Spartan John-117 has been in cryo-sleep, adrift, while his ancillary AI, Cortana, has been trapped in her own thoughts. She had been looking for a reason -- any reason -- to wake the SPARTAN up. Now she had one.

They were crashing.

{Description Edited 4/25/2014: Clarified Installation 04B}
{Edit 05/02/2014: Trimmed character list pending future change}