Half-Life: Original and Sequel

Half-Life

At the underground Black Mesa Research Facility, the theoretical physicist Gordon Freeman participates in an experiment on a crystal of unknown origin. This triggers a "resonance cascade", which greatly damages the facility and teleports in hostile alien creatures. Eli Vance tells Freeman to get to the surface and find help. After going through several layers of Black Mesa, Freeman discovers that United States Marines and black operators have been dispatched to the facility to terminate the aliens but also to cover up the incident by eliminating all the remaining Black Mesa personnel. A scientist instructs him to make his way to the Lambda Complex to stop the alien invasion.
Freeman then discovers a monster called the Tentacle, and kills the giant creature using a rocket engine test facility that incinerates it, and uses an underground monorail to reach a rocket silo. He launches a satellite to help the Lambda team, but is captured by vengeful Marines and left for dead in a trash compactor. Escaping through a waste treatment complex, Freeman travels through a part of Black Mesa filled with alien specimens, collected long before the resonance cascade.
Overpowered by the aliens, the Marines withdraw and began to launch air strikes. Freeman crosses the facility's surface to reach the Lambda Complex, where he discovers secret teleportation technology. There, scientists inform him that a powerful alien creature is preventing them from closing the portal. They teleport him to the alien dimension Xen to kill it. There, Freeman encounters dead scientists who teleported to Xen before him. He kills a large alien, the Gonarch, and finds a factory that manufactures alien soldiers. Finally, he enters the Nihilanth's lair and kills it. Freeman is detained by the G-Man, a mysterious agent who claims his "employers" wish to hire Freeman. If he accepts, Freeman is placed into stasis; if not, he is teleported to his death.

Half-Life 2

After the alien invasion at the Black Mesa Research Facility, a multidimensional alien empire known as the Combine has conquered Earth. Approximately twenty years after being placed in stasis, Gordon Freeman is inserted into a train bound for City 17 by the G-Man (Michael Shapiro). Helped by the undercover Resistance member Barney Calhoun (Shapiro), Gordon attempts to reach the laboratory of Dr. Isaac Kleiner (Harry S. Robins) but is subdued by Combine officers. He is rescued by Alyx Vance (Merle Dandridge), who guides him to the laboratory. Kleiner's attempt to teleport Gordon to the Resistance base fails, and Gordon is momentarily teleported to the Citadel, the skyscraper headquarters of Dr. Wallace Breen (Robert Culp), the former Black Mesa administrator and the Combine's puppet ruler. Gordon uses an airboat to progress to the base via the city's canal system, eluding Combine forces.
At the base, Gordon reunites with Alyx's father, Dr. Eli Vance (Robert Guillaume), and meets Dr. Judith Mossman (Michelle Forbes). Alyx introduces Gordon to her pet robot, Dog, and gives Gordon the gravity gun. Combine forces storm the base and capture Eli and Mossman. Gordon diverts through the zombie-infested town of Ravenholm, aided by its lone inhabitant, Father Grigori (Jim French), to a Resistance outpost whose occupants provide him with a dune buggy. He drives along the coastline of depleted water levels to reach the Combine prison of Nova Prospekt, where Eli and Mossman are being held. Gordon and Alyx reunite, locate Eli, and discover that Mossman is a Combine spy. Before they can stop her, Mossman teleports herself and Eli to the Citadel. The teleporter explodes moments after 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, during which time the Resistance has used the attack on Nova Prospekt to launch an armed rebellion against the Combine. Aided by Barney and Dog, Gordon fights his way to the Citadel. Inside, a security system vaporizes his weapons but inadvertently super-charges the gravity gun; this helps Gordon climb the tower. He is captured and taken in a transport pod to Breen's private office, near the Citadel's apex, where he and Mossman are waiting with Eli and Alyx as captives. Breen reveals that he plans to use them as leverage to negotiate with the Combine, contradicting what he had told Mossman. Angered, Mossman frees the captives and Breen attempts to escape using a teleporter. Gordon destroys the reactor and Breen falls into the abyss. Moments after the reactor explodes, the G-Man reappears and freezes time, praises Gordon's actions, and returns him to stasis.