Half-Life 2: Episodes 1 & 2

Half-Life 2: Episodes 1 & 2 are the first two parts of the Half-Life 2 sequel. Consciousness returns. Gordon and Alyx discover they've somehow escaped both the reactor explosion and the G-Man's malevolent grasp. Before they can ponder their miraculous survival, the crippled Citadel lurches back to life just long enough to initiate a self-destruct sequence. With Alyx at his side, Gordon must flee a city in chaos before the Citadel's final detonation turns City 17 into a toxic, mutant-infested crater. In Episode Two, you must battle and race against Combine forces as you traverse the White Forest to deliver a crucial information packet stolen from the Citadel to an enclave of fellow resistance scientists.

The episodes share the exact same codebase (referred to as "episodic"). It's built off of Half-Life 2's code. Some episodic things were backported to HL2, though most were not, to ensure that the new code didn't mess up anything in HL2.