You were at Nova Prospekt with Gordon and Alyx, your mission was to save Eli and find Mossman, but then something shows up on a Combine monitor and there is Mossman on the screen

Alyx: What’s she doing?

Dr. Breen is heard on the monitor but not seen

Mossman: You said you wouldn’t touch Eli…

Dr. Breen: I can admit, the soldiers were a BIT overzealous, but he was too good of a catch to let go. Especially since he was housing Gordon Freeman.

Mossman: You would have had Freeman if you waited for my single!

The video on the screen ends

Alyx: DAMN HER! Calms down Alright you two, we need to find her.