Welcome to Black Mesa East, the fifth level of Half Life 2.
Meet The Team
You'll land after quite a long drop in the waters on the other side of the dam. Drive up to the lambda symbol outside the rather large complex on your left, and get out of your airboat. Unfortunately, this is where you and your trusty airboat must part, so if you wish to say your goodbyes now is the time (don't worry, although you're still really quite close to a Combine patrol area nothing will ever come by this way, so you're safe).
Head up the ladder and onto dry land. Go under the power pylon and up the ramp past the old trailer. Past the barrels and the graffiti on the walls, go inside the fenced section and past the barrels into the building entrance lit up ahead. All of a sudden everything will go dark, before being lit up with the lights of scanners. You'll be identified as safe, and greeted by Dr. Judith Mossman, another scientist working with Eli, Kleiner and the resistance. She'll take you to the lift which leads to where Eli is working, and explain a little bit about their current ventures in teleportation.
When the lift reaches the bottom you'll be introduced properly to Eli Vance, and you'll get to look around his lab section — there are various things which prompt a little extra exposition from him, such as looking at the "Earth Surrenders" headline on the noticeboard, or the picture of Eli, his wife Azian, and Alyx as a baby, and even the weird thing in the green jar. After a while Alyx will come in via one of the doors and say hello. You'll have to sit through some character development between everyone before you and Alyx will head over to another section of the facility where you will get a new tool, the Gravity Gun!
The next part with you, Alyx and Dog basically serves as a little training in how to use the Gravity Gun, since the next chapter of the game, and various places throughout the rest are filled with opportunities to use it, or where it's otherwise necessary to conserve ammo. Alyx will first describe the primary fire, which is used to repel an object or small headcrabs (but not Combine soldiers). She will then describe the secondary fire, which allows you to pull objects. If you haven't done them already, she will prompt you to pull barrels from the left shelf, then stack wooden boxes to the upper path. Dog will also play catch with you; the only requirement is to catch three wooden crates that are thrown at you; after this is free play with the ball.
Ultimately, though, the rest of the game would be quite boring if you were only helping out with experiments, and sure enough, the Combine begin sweeping the area shortly after you've finished playing fetch with Dog. Run back to the airlock you came through, and you'll make your way back to the lab where Eli was working. However, the ceiling caves in between you and Alyx, and you and Dog are trapped on the wrong side. Alyx will tell you to go through Ravenholm (despite Eli trying to tell you not to go that way) and Dog will open up the gate. Head through the junk-riddled tunnel to the mineshaft, knock the padlock off the ladder grate with the crowbar, then climb up and go around the corner to enter the zombie-infested village of Ravenholm.