Whilst in City 17's underground rail network the duo are faced with many enemies including the Zombine, a cross between the normal headcrab zombies and Combine soldiers. Unfortunately the train derails on the way to their destination and and the pair are forced to walk the rest of the journey.

Using the Citadel train system and having downloaded all the required information, Gordon and Alyx make a hasty escape from the Citadel.

Alyx with an information screen featuring Isaac Kleiner behind her She also finds a message by Judith Mossman describing a new discovery but is stopped after alerting the attention of Combine Advisors. She finds out that the Combine deliberately sped up the core meltdown so they are able to send a message back to Combine homeworld. While still inside the Citadel, Alyx goes delving through the Combine's computer to uncover more information on their plans. The unstable Citadel CoreДЄlyx and Gordon go back into the Citadel to stabilize the core and after several fire fights with the remaining Combine soldiers are successful in their cause. After regaining consciousness outside the Citadel, Gordon reunites with Alyx and Dog and, after talking to Drs Isaac Kleiner and Eli Vance, must re-enter the Citadel and slow down the explosion of the central core which would be catastrophic. Things don't go the G-Man's way after Vortigaunts appear to save Alyx and then take Gordon out of stasis. After the explosion of the reactor in the Citadel, Gordon Freeman is once again put into stasis by G-Man. Story Alyx frozen in timeД®pisode 1 begins immediately after the events of Half-Life 2. Miraculously plucked from the massive explosion of the Combine Dark Energy Reactor by Vortigaunt allies, Gordon and Alyx Vance are tasked with temporarily stabilizing the Combine Citadel's unstable energy core before it detonates and escaping from City 17 before The Citadel explodes. Players once again fill the shoes of Gordon Freeman, and the action picks up not long after the finale of Half-Life 2.

Half-Life 2: Episode One is the first episodic installment of Valve's Half-Life franchise, and the second release in the Half-Life 2 saga.

Half life 2 episode one walk through