Page "Gordon Freeman" Paragraph 13

Half-Life 2: Episode One rejoins Gordon and Alyx Vance, who are separately removed from the G-Man's stasis, and rescued a split instant before the reactor explosion, by the Vortigaunts, greatly displeasing him.

Gordon regains consciousness under a pile of rubble and is found by Alyx and Dog.

Their proximity to the Citadel, coupled with its imminent explosion, which would level much of City 17, requires them to go back inside and stabilize the core, stalling the structure's destruction long enough for both themselves and much of the human population to escape.

Gordon and Alyx succeed in doing so, but learn that the local Combine forces are attempting to send a distress message for off-world assistance, using the Citadel's destruction to power the transmission.

The Combine consider this a positive, as the subsequent explosion would destroy all of City 17 and much of the surrounding countryside, which has been all but lost to human resistance forces.

With a copy of the distress message, Gordon and Alyx escape the Citadel and meet up with Barney and other survivors.

The pair escape City 17 via an evacuation train as the Citadel core goes critical, sending out the Combine message.

The resulting shockwave derails their train.