The expansion takes place right after the events in HL2, and you
and Alyx will flee from City 17, fighting against creatures who
were previous blocked from enetering the city. The Citadel you
assaulted in HL2 is tumbling into chaos, and this event will have
catastrophic effects on the surrounding area, prompting your
escape. Secondary characters Alyx and Dog will have a much more
significant role in this follow-up.
check out a screenshot here.
http://media.pc.gamespy.com/media/740/740555/imgs_1.html
