Much of the backstory of HL2 can be found in game, but often you must activate NPCs to tell you, such as looking at the newspaper cuttings in Klieiner's lab (Red Letter Day), which themselves are helpful also. There is comparatively little plot in HL1, all it tells you about are the major events that led to the Xenian creatures (headcrabs, Vortigaunts, barnacles, ichthyosaurs, and others cut from the game. Presumably also antlions) arriving on Earth and the destruction of the Nihilanth and its forces (freeing the alien slaves, enemies in that game that turn out to be Vortigaunts). What plot there is is largely unrelated to HL2, such major development takes place in the 20 or so years that pass while the G-Man has Gordon "employed" in stasis. The Combine are new to HL2.
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