Half Life 2

You can go for the Alliance or the Resistance

Despite the brutal terror, an underground Resistance (rebels) operates on Earth. It is led by former Black Mesa scientists Eli Vance, Isaac Kleiner, and Eli's daughter, Alyx Vance. Their main advantage is that they have developed local teleportation technology, which the Combine lacks (the invaders can travel between universes, but are poor at traveling within a single planet). The rebels are waiting for the spark to start a revolution, and that spark is the sudden reappearance of Gordon Freeman, who is brought out of suspended animation by the mysterious G-Man 20 years after the events of the first film.

Social and Military StructureCivil Defense (GO / Civil Protection): Human volunteers. They joined the occupiers for food, shelter, and security. They serve as a brutal police force. Combine Soldiers (Combiners): Humans who have undergone radical surgical and cybernetic modification. Their memories are erased, their emotions are removed, and their organs are replaced. They are transformed into obedient living weapons. Synthetics: Living creatures from other planets, which the Combine has transformed into combat machines (huge three-legged Striders, flying Stormtroopers, and Hunters). Vortigaunts: Former enemies of humans from the first game, a race of alien slaves. After the death of their master (Nihilanth), they gained freedom, fled to Earth, and became staunch allies of humanity in the fight against the Combine.

New World StructurePrison Cities: The surviving population is herded into numbered cities. The game's story begins in City 17, an Eastern European metropolis that has become the capital of the occupation regime. The city's center is the Citadel, a gigantic Combine skyscraper several kilometers tall.Birth Suppression: The Combine has activated a global "suppression field." It blocks DNA strands, making conception impossible. For 20 years of occupation, not a single child has been born on Earth. Humanity is doomed to slow extinction.Biosphere Destruction: The invaders are actively draining Earth's resources. Ocean levels have plummeted, water is being replaced by toxic waste, and terrestrial fauna is being virtually wiped out by predatory parasites from Xen (parasitic crabs called Headcrabs and Antlions). Personality Erasing: Drugs are added to the water that gradually erase people's memories, so they forget their lives before the occupation and avoid rebellion.

Timeline: How Earth Came to DisasterResonance Cascade (Half-Life 1): In 1998, scientist Gordon Freeman accidentally opens an interdimensional rift at Black Mesa. Aggressive creatures from the world of Xen begin teleporting to Earth.Portal Storms: The disaster cannot be contained. Portals open all over the planet. The terrestrial ecosystem is destroyed, and humanity hides in large cities under military protection.Seven-Hour War: Attracted by powerful energy surges, the Combine army—a highly technological invader race—arrives on Earth. They destroy all armed forces on the planet in just seven hours.Surrender: Former head of Black Mesa, Wallace Breen, contacts the invaders and signs an act of complete surrender on behalf of humanity. The Combine appoints him Administrator of Earth (viceroy).

Returning to Dr. Kleiner's lab, Gordon and Alyx learn that a full-scale Resistance uprising has been underway for a week. With the help of Barney and Dog, Gordon breaks into the Citadel, and Alyx is captured. Inside the Citadel, an energy field disarms Gordon while simultaneously enhancing his gravity gun, and he manages to break through to the top of the tower. Gordon is captured and transported to Dr. Breen's office in a transport pod. Dr. Breen reveals that he intends to use the prisoners as leverage during the Resistance's negotiations with the Combine, contradicting his earlier statement to Judith.[4] An outraged Judith frees Gordon, Alyx, and Eli. Dr. Breen rushes to the teleporter on the roof of the Citadel, which is linked to the Combine world, but Gordon destroys the Citadel's reactor, and Dr. Breen falls into the abyss. Just as the reactor explodes, the G-Man reappears, freezes time, thanks Gordon for his work, and places him back into stasis.
The world of Half-Life 2 is a dark dystopia in which Earth is under the complete control of a totalitarian intergalactic empire called The Combine.

Arriving at Black Mesa East, Gordon meets former Black Mesa employees Eli Vance (Alyx's father) and Judith Mossman. Alyx introduces Gordon to her pet robot, Dog. Gordon plays with Dog and the gravity gun, but Black Mesa East is suddenly overrun by Combine forces, and Eli and Judith are captured. Gordon is forced to flee through the zombie-infested ghost town of Ravenholm. The town's only surviving resident, Father Grigori, helps him reach the mine. After escaping the town, Gordon reaches a Resistance outpost and travels by buggy along Route 17 to Nova Prospekt Penitentiary, where Eli and Judith are being held. He infiltrates the prison and meets with Alyx. They find Eli and learn that Judith is a Combine spy. Before they can stop her, Judith teleports herself and Eli to the Citadel. Gordon and Alyx escape Nova Prospekt via the teleporter.

Following an alien invasion of the Black Mesa research facility, an alien civilization known as the Combine conquered and subjugated Earth during the Seven Hour War. Approximately twenty years after the events of the first Half-Life, Gordon Freeman awakens from stasis to the voice of the G-Man, informing him that his "time has come again." Gordon awakens on a passenger train arriving in City 17. This city is home to the Citadel, a multi-kilometer-tall structure that serves as the Combine's headquarters on Earth. With the help of Resistance member Barney Calhoun, Gordon travels to Dr. Isaac Kleiner's laboratory, but is detained and beaten by Combine soldiers along the way. Alyx Vance comes to Gordon's aid and takes him to the laboratory. Dr. Kleiner sends Alyx, followed by Gordon, to Black Mesa East, the Resistance's main base, but during Gordon's teleportation, the headcrab Lamarr disrupts the settings. Gordon briefly teleports to various locations, including the Citadel, to the office of Dr. Wallace Breen, the former administrator of Black Mesa, who became the puppet temporary administrator of Earth after the Combine invasion. The teleport drops Gordon near the laboratory, Combine forces raise an unprecedented alarm, and Gordon faces a long and dangerous journey to the base.

The action takes place approximately 20 years after the events of the first game. During this time, Earth has been captured by an alien civilization—the Combine—during the Seven Hour War. The enigmatic G-Man awakens Gordon Freeman from stasis and places him on a train bound for City 17, the home of the Citadel, the Combine headquarters.
In the city, Gordon encounters a totalitarian regime. With the help of Barney Calhoun, a former Black Mesa security guard and Resistance member, he attempts to reach Dr. Isaac Kleiner's laboratory. After a series of adventures and a meeting with Alyx Vance, Eli Vance's daughter and fellow Resistance member, Gordon finds himself at Black Mesa East, the Resistance's main base. However, during the teleportation, Kleiner's pet headcrab, Lamarr, disrupts the settings, and Gordon ends up in a conduit.