Gordon Freeman was not the only one under contract with GMan. There were five others along with him. You are one of them, part of the elite "Eridani" squad. Epsilon Eridani is a single-player mod for Half-Life 2, set during the actual story of Half-Life 2. EE's story is divided into three parts. The first part takes place during the 7-Hour War, the second around two weeks before HL2 and the third during HL2. In EE you play Epsilon, one of the five Eridanis, a group of individuals selected by the GMan to help the rebellion in its struggle against the Combine.
This is EE main theme music, sorry for the bad audio quality, I sampled this one at around 48kbps to have a low file size, but the one that will be featured in the mod is 320kbps and 24Bit crystallized.