Half-Life 2: Episode One is a direct continuation of the story of Half-Life 2. Similarly, HL2: Episode Two continues where Episode One leaves off. Valve origin

Half-Life 2: Episode One is a direct continuation of the story of Half-Life 2. Similarly, HL2: Episode Two continues where Episode One leaves off. Valve originally planned on releasing a series of episodic entries in the HL series, two per year if I remember correctly. However they were never able to meet this schedule.