You play as one of the human rebels - Frank Harrison, who's currently in his hideout outside City 17. He's woken up by the Citadel explosion, caused by Dr. Freeman and Alyx Vance at the end of Half-Life 2. The evacuation of the City 17 is in progress and he has to leave his house to fight his way back to City 17 to join an evacuation train while there's still some time. Gather your supplies and an old HEV suit and explore the territories of the City 17. Play your role in the events, that took place during the original Half-Life 2: Episodes 1 and 2.
The best mod I have ever played involving Half-Life 2's storyline. I highly recommend it.