It starts with a wind and strings melody variation of the Gregorian chant of the Halo Theme. Later, deeper strings join in and take on the main melody, which is instantly recognized as Dust and Echoes from the ending of Halo: Combat Evolved. At 1:45, the song changes to a minor key, only to be followed by a major chord at the end.
The name of the song is derived from the final words of John-117 in Halo 3: "Wake me... when you need me."
It runs for 2:19.