Pretty long read; Stuff goes to sh*t at a facility called "Black Mesa" The protagonist called Gordon Freeman was the rat lab there. So he tries to escape and the military is after him. Along the way he finds aliens that try to kill him. He goes to a dimension where the leader of the aliens are and destroys them. Half Life 2 is in a city called "City 17" and the combine (enemy) are also after him so he and a team called the "Resistance" is going to destroy a tower which is the base of the combine. They destroy it and stuff also goes to sh*t (surprise?) The combine are pissed and sends heavy force for the Resistance and they launch a missile at the remains of the combine and YAY! They win.... Someones father dies and that's where it ended. You could search up a better explanation but that's kinda all you needed to know.