Half-Life 2: Episode One is the first of the two episodes that take place after the end of Half-Life 2.
After an ending oddly reminiscent of the first game, we are once again left on a cliffhanger after the events of Half-Life 2. This game is the continuation of Half-Life 2, and once again we take control of Gordon Freeman on his unstoppable killing spree.
While Episode One is very short in comparison to Half-Life 2, it still holds the same level of quality in terms of gameplay. The same characters are there, and a few new enemies and mechanics are introduced. None of this is exactly game-changing, yet it does keep it fresh. The story lacks a little, which is mainly due to the lenght of the game.
There isn't much to say for Episode One that hasn't already been said for Half-Life 2. While it is fun to fight alongside Alyx, it doesn't quite make up for the length of it. If purchased through the Orange Box, it isn't much of a problem.
Overall, definitely worth playing, but only as part of the series, and not on its own.