We emerge at the top of the citadel, where Half-Life 2's plot ended. A second before the core explodes, the Vortigons appear and save Alyx and me, stopping time and teleporting us to safety. Afterward, we see the Vortigons shielding us from the G-Man, meaning we're now on our own.
The Mechanical Hound digs us out of the rubble and hands us the gravity gun. We talk to Alyx; she doesn't remember anything about teleporting. We need to get out of the pit. We hit the fence boards with the gravity gun, causing the chain-link fence to fall, and we climb up it.
We talk to Eli and Isaac on the computer. We learn that the citadel is about to explode. We can slow down the core's reaction to allow us time to escape the city, but to do that, we need to return to the citadel.
We arrive in a room with electric doors. Crawl to the left, jump down along the platforms, and press the red button. Catch one of the round mines and return to the top via the elevator. Hold the mine in front of Alyx; she'll reset it. After that, throw the mine into the room with the stalkers on the right. After they're dead, we can unlock the door inside.
We see white combines ahead. Crawl to the left through the rubble-strewn corridor. We destroy the next rubble with the gravity gun. Jump over the railing.
You need to run along the catwalk, where a gravity field activates on the side. Wait for the field to activate and then deactivate, then quickly run across before it activates again.
The next room has weapon-destroying devices on the walls. The device is destroyed by the gravity gun. This causes the gun itself to become much more powerful and turn blue.
Ahead is an energy bridge, only half-powered. We see an energy ball at the top; we attract it and throw it into the receiving device on the lower left. When the ball hits the device, the bridge will activate, allowing us to proceed.
Ahead, we fight Combines. With the new blue gravity gun, we can attract not only small objects but even people. We throw Combines at each other or hurl them against walls.
Near the third bridge, it's the same story, but the left device is covered with armored glass. To throw the ball there, aim slightly higher, at the wall above the glass; the ball will ricochet and land where it's intended. We run along a narrow path and jump over a chasm over a broken glass floor.
The elevator stops. We see the receiver behind glass to the side. To the right of it, we rip out two panels. We take the ball and throw it into the open corridor on the right; the ball will ricochet into the device. We deflect another falling piece of debris.
The elevator stops again. The device is blocked from the side by a paneled pillar. We rip out the control panels on the left and right. We launch the ball just to the right of the pillar, and it will hit the device. A stalker will emerge and deactivate the device.
We reach a stop. A huge piece of debris begins to fall from above. We quickly exit the elevator, running across the catwalk. The elevator will collapse, and we continue down the floor. Now on to the next chapter.