Half-Life 2 computer game series

The Half-Life 2 computer game series is set on a future Earth that's been taken over by an alien group known as The Combine from another dimension, elsewhere in the universe or even another universe. This invasion occurred after a portal was opened into that dimension or other place during the action in an earlier game called Half-Life (1998). Half-Life 2 was released in 2004 and a subsequent trilogy of episodes started appearing in 2006 (Half-Life 2: Episode One) and 2007 (Half-Life 2: Episode Two). Half-Life 2: Episode Three has not been released as of August 2009.