Awesome sequel to the original!
Half-Life 2 begins with you starting out in a city called City 17, that is controlled by an authoritarian regime under the direction of what seems like an alien enemy known as the Combine. Your journey begins on a train that stops at a station surrounded by guards of the regime. From their you make your way to a location all the while trying not to draw attention to yourself so you don't draw the attention of the regime. There is a underground movement encompassing many of the characters from the original Half-Life. Without giving away too much, you play your way through the game fighting this authoritarian regime with the help of an undergound movement as well as 'other's' (without giving away too much) intent on toppling this regime.
Half-Life 2's gameplay is an amazing tour-de-force. The action is well paced, mixed with well timed puzzles set out through the game. I feel like I am on the move all the time with an atmosphere that is tense due to the many henchman of the regime and not to mention the many robots that fly around tracking down any rebels and all those who oppose the regime. You meet various characters in the game, that will assist you in your quest to topple this authoritarian regime. The game keeps the action moving forward with an engaging storyline that is unmatched for a game released in it's time.
With respect to graphics, it is easily one of the best looking games from 2004. While it may not beat the graphics in Doom 3 that came out couple of month's before, the facial animations of the characters is excellent that does an excellent jobs of conveying the emotions of the characters in the game. The environments and settings in the game is also well constructed. The physics in the game is also excellent for a 2004 game.
In summary, Half-Life 2 raises the bar in the First Person Shooter genre just like it's predecessor. It is quite possilby the best First Person Shooter of it's time. It's a game that no First Person Shooter should miss!