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From HalfLife 2 Knowledge Base

You arrive at Black Mesa East on your airboat. Once you find your way inside, the team notices activity in the airlock and investigate to see who, or what, it might be. They find you, Gordon Freeman, standing there, and decide to decontaminate you out of precaution.

One of the first people you encounter in Black Mesa East is Dr. Mossman. She showers praises on you, telling you how much of an honor it is to be working side-by-side with you. After this show of affection, she leads you to an elevator where you see various levels of activity by the Vortigaunts on your way down to Dr. Vance's lab.

Once in the lab you're invited to look around as you wait for Alyx to come in. Eventually Alyx will come, and Dr. Vance suggests that she show you how to use the gravity gun for the first time. Dr. Mossman insists that the gravity gun is NOT a toy, but Alyx simply shrugs it off and leads you to a practice ground of sorts, past the old tunnel to Ravenholm.

Once you receive the gravity gun, Alyx will prompt you with instructions and small challenges to get you familiar with the weapon. After the short introduction to the gun, you're introduced to DOG, Alyx's dog-like robot. You can play a game of catch with DOG with his special ball and your gravity gun. After some time, scanners will start sweeping the area, which causes Alyx and DOG to run for cover. You'd do well to follow suit. Back inside the system of tunnels, a rockslide breaks you and Alyx apart, but DOG is still by your side. Alyx instructs DOG to open the tunnel to one of the creepiest parts of the game... Ravenholm.