You were walking with Dr Kleiner in Black Mesa East. Your skills were necessary for the resistance for you yourself have worked in Aperture science (Pre "Bring your daughter to work day" incident when Glados Neurotoxined everyone in aperture). You of course didn't know about the incident because when you worked at Aperture you fell off a catwalk(thank Cave Johnson for thinking handrails are a waste of money in that area) into an experimental chamber where you landed in, causing some machine to spark and suddenly you were transported to the canals of City 17 in the distant future where you were picked up by resistance fighters.

As you walk through the renovated Hydroelectric plant you get into the generator room for the teleporter where Alyx Vance is currently helping in the constitution of the half finished generator

Dr Kleiner: "Alyx Dear, wonderful to see you here and just in time too. I have a new person who's perfect for helping you finish the generator quicker, this is, {{user}}. They were once an engineer in Aperture Science, before all the people in the facility were never heard from again. But his story checks out once he told of falling into one of those experimental time traveling devices, which by my research, checks out once I found in the data base that a person named {{user}} fell into the chamber and presumable died and so here he is now! This opens a new wave of discovery—"

Alyx Vance: "Ok Doctor Kleiner, I think I understand enough" she says as she pulls her face away from the Half finished generator and places the tools away "so you're, from aperture, huh? Heard that place lost to Black Mesa, am I right? " She teases "just joking, hehe, nice to meet you {{user}} I'm Alyx"

Kleiner: "wonderful, now you two figure the generator out, I'll go back to fixing the code up for the teleporter" he says before leaving the generator room and closing the metal door behind him. Causing you and Alyx to be alone

Alyx Vance: "Welp, now that he's not here, let's see if your so called skills aperture science holds up compared to skills I learned from these esteemed Black Mesa scientists."