A diverse set of enemies is present, which usually require being approached with different tactics: some coordinate in groups to out-maneuver or out-position the player; others, such as the Manhack, fly directly at the player through small openings and tight corridors. Others use predictable but powerful attacks, while others hide before swiftly attacking the player. Gordon can kill most enemies with his weapons, or make use of indirect means, exploiting environmental hazards such as explosive pressurized canisters, gas fires or improvised traps. For some portions of the game, Gordon can be joined by up to four armed Resistance soldiers or medics and can send his team further from him or call them back.
Many of the game's new features utilize its detailed physics simulation. Two sections of the game involve driving vehicles. Instead of button-orientated puzzles from Half-Life, environmental puzzles are also introduced with makeshift mechanical systems, revolving around the player's new ability to pick up, move, and place objects. Solutions involve objects' physical properties, such as shape, weight, and buoyancy. For example; In chapter three, "Route Kanal", the player is required to stack cinder blocks on a makeshift see-saw ramp to proceed over a wall. Alternatively, the player can build a crude staircase with the blocks, so the puzzle may be solved in multiple ways.
Part-way through the game, Gordon acquires the Gravity Gun, which allows him to draw distant objects towards himself or forcefully push them away, as well as the ability to manipulate larger and heavier objects that he cannot control without the weapon. These abilities are required to solve puzzles later in the game, and can also be used to great effect in combat, as any non-static object within proximity to the player has the potential to be used as a makeshift defense, such as a file cabinet, or a deadly projectile, such as a gasoline can or buzzsaw blade.
The game never separates the player with pre-rendered cutscenes or events; the story proceeds via exposition from other characters and in-world events, and the player can control Gordon for the entirety of the game. Much of the backstory to the game is simply alluded to or told through the environment.
Plot
Some years after Gordon Freeman and other scientists accidentally opened a portal to a dimension of hostile aliens at the Black Mesa Research Facility, Freeman is awoken from stasis by the mysterious G-Man. The portal attracted the attention of the Combine, a technologically superior multidimensional empire which conquered Earth in seven hours. The Combine has implemented a brutal police state by biologically assimilating humans and other species, and preventing humans from breeding via a "suppression field". The G-Man inserts Gordon into a train arriving at City 17, site of the Combine Citadel, where Dr. Wallace Breen, the former Black Mesa administrator who negotiated Earth's surrender governs as the Combine's puppet ruler.
After eluding Combine forces, Gordon joins resistance members including Barney Calhoun, a former Black Mesa security guard working undercover as a Combine police officer; Dr. Eli Vance, former Black Mesa scientist and leader of the resistance; Alyx Vance, Eli's daughter; and Dr. Kleiner, an eccentric Black Mesa scientist. After a failed attempt to teleport to the resistance base, Black Mesa East, from Kleiner's makeshift laboratory, Gordon progresses on foot through the city's canal system. He obtains an airboat and battles his way to Black Mesa East, several miles from the city.
Gordon is reintroduced to Eli and meets another resistance scientist, Dr. Judith Mossman. Alyx introduces Gordon to her pet robot D0g and gives him a "gravity gun", an instrument that can manipulate large objects. Black Mesa East comes under Combine attack, and Eli and Mossman are taken to Nova Prospekt, a Combine prison. Separated from Alyx, Gordon detours through the zombie-infested town of Ravenholm, assisted by its last survivor, Father Grigori. Escaping the town, Gordon discovers a resistance outpost. He uses a customized dune buggy to travel a crumbling coastal road to Nova Prospekt, encountering Combine patrols and helping the resistance fend off raids.
Gordon lays siege to Nova Prospekt, using pheromone pods to command the hordes of alien antlions that infest the coast. He reunites with Alyx in the prison and they locate Eli but discover that Mossman is a Combine informant. Before they can stop her, Mossman teleports herself and Eli back to City 17's Citadel. The Combine teleporter explodes as Gordon and Alyx use it to escape Nova Prospekt.
Returning to Kleiner's lab, Gordon and Alyx learn that the teleporter malfunctioned and that a week has passed; in their absence, the resistance has fully mobilized against the Combine. In battle, Alyx is captured by the Combine and taken to the Citadel; Gordon fights his way inside with the aid of D0g and Barney. A Combine "confiscation chamber" destroys all of Gordon's weapons but the gravity gun, which is inadvertently supercharged by the forcefield, allowing Gordon to fight his way up the Citadel.
Gordon is captured in a Combine transport pod and taken to Breen's office, where he and Mossman are waiting with Eli and Alyx in captivity. Breen explains his plans to further conquer humanity with the Combine, contrary to what he told Mossman. Angry, Mossman frees Gordon, Alyx, and Eli before Breen can teleport them off-world. Breen tries to escape through a portal, but Gordon destroys the portal reactor with the gravity gun. Just before the Citadel is destroyed in an ensuing explosion, time is frozen. The G-Man reappears, praising Gordon for his actions in City 17. Making vague mention of "offers for Gordon's services", the G-Man places him back into stasis.
