Hλlf-Life is a series from Valve, beginning in 1998 receiving two expansions (Opposing Force and Blue Shift in 1999). Hλlf-Life 2 was released in 2004, with Hλlf-Life 2: Episode 1 in 2006, and Hλlf-Life 2: Episode 2 in 2007, with the story never being finished for 12 years. 

In Half-Life, the player controls Gordon Freeman, a scientist working in the Black Mesa research facility, with an experiment leading to an alien invasion and the deaths of multiple personnel, with the player being hunted by these aliens and the U.S. military while they try to escape, with Freeman being put into stasis in the end for reuse later.

Half-Life Blue Shift follows a Black Mesa Security officer named Barney Calhoun, and Half-Life Opposing Force Follows HECU Corporal Adrian Shepherd, both following a similar story to Half-Life.

Half-Life 2 and its episodes follow Gordon Freeman once more many years after Half-Life, with the earth being under alien control due to Half-Life’s events. Gordon Freeman is awoken to defeat the invaders by joining the human and alien resistance, and assisting them in victory.

The Half-Life series has never been expanded to the confirmed Half-Life 3 and Half-Life 4.