covering fire to keep the player safe while they travel to a certain area or perform certain actions. Plot In City 17, Gordon Freeman and his ally Alyx Vance have destroyed the reactor at the heart of the Combine Citadel. As it explodes, the G-Man intervenes and extracts Gordon. Several Vortigaunts appear and rescue Alyx from the blast. The Vortigaunts appear before the G-Man and stand between him and Gordon. They teleport Gordon away from the scene, restricting the effect of G-Man's abilities on Gordon. Alyx's pet robot Dog retrieves Gordon from rubble outside the Citadel, and Gordon reunites with Alyx. Alyx contacts her father Eli Vance and scientist Isaac Kleiner, who have escaped the city. Kleiner tells them the Citadel's core will soon explode, which will completely wipe out City 17. Alyx and Gordon re-enter the Citadel and temporarily stabilize the core. Alyx discovers that Combine Elite soldiers are accelerating the destruction of the Citadel to send a "transmission packet" to the Combine's homeworld. She downloads a copy of the message, along with a transmission from Dr. Judith Mossman, in which she mentions a "project" she has located, before she is cut off by a Combine attack. Alyx and Gordon board a train to escape the Citadel, but it derails. They proceed on foot in the city streets, fighting through the disorganized Combine forces and rampant alien infestations. They meet Barney Calhoun and a group of other survivors who are preparing to move to a train station to escape City 17. Alyx and Gordon lead and escort the survivors to the station, as the way there is constantly being swarmed and sabotaged by Combine forces. Alyx and Gordon take a different train out of City 17, escaping just as the reactor detonates and the Combine's message is transmitted. Several rocket pods containing Combine Advisors launch from the Citadel as it detonates. The resulting shockwave derails the train. Development Valve developed Half-Life 2 (2004) over six years using its new game engine, Source. In April 2005, Valve announced an expansion for Half-Life 2 under the working title Aftermath. Designer Robin Walker said: "Right now, we're really, really good at making Half-Life 2 ... We'd just got comfortable with all our tools and what we could do. That's normally the point in which we'd go off and make new tools — we didn't want to do that." Valve president Gabe Newell