In "", the destruction of the teleporter at the top of the Citadel has isolated the Combine forces on Earth, but also caused its primary reactor to begin to melt down. This forces Gordon Freeman and Alyx Vance to journey back into the critically damaged Citadel to temporarily stabilize its reactor while the city's inhabitants are evacuated. The Combine forces, however, instead attempt to accelerate the meltdown in order to send a message to their native universe for reinforcements. After Alyx acquires an encrypted copy of the message to be sent, Overwatch forces desperately attempt to stop the pair from escaping the city, spurred on by Combine Advisors. As the pair escape on a train at the end of the game, the Citadel detonates, sending the message and destroying City 17.