It has been five years in the making, but the sequel to the hugely popular Half-Life game will be worth the wait, say its makers.
The alien hordes return in the sequel
Valve say Half-Life 2 will do as well as the original, which won many awards and sold more than five million copies.
"We're confident that we haven't wasted the last five years," said Valve's Director of Marketing, Doug Lombardi.
Valve says Half-Life 2 is due to be shipped by the end of September, despite rumours of a delay.
Half-Life has been one of the most popular and widely acclaimed games of all time.
At the time of its release in 1998, it was praised for its strong plot, complex puzzles and the intelligence of the enemies in the game.
The title went on to pick up more than 50 game-of-the-year awards and sell millions.
It has stayed alive in the minds of gamers, thanks to add-on adventure packs such as Blue Shift and the popular multi-player game Counter-Strike.
"After Half-Life 1 we had a lot of ideas about where we wanted to go," said Mr Lombardi.
"The success of the game and add-ons like Counter-Strike allowed us to chase our wildest dreams."
The team at Valve has five years working on Half-Life 2, with an average of 30 people working on the title at any one time.
The sequel continues the story of laboratory assistant Gordon Freeman, who once again has to take up the fight against an invading alien horde.
But Valve wanted to do more than just another shoot 'em up. As with the original, it has focused on a strong plot to keep the player engaged.
"There are a lot of great action games but what is interesting for us is story-telling, making connections with a player," said Mr Lombardi.
"We're pushing in different directions. We're trying to compete with films and TV and other art forms, rather than games."
He said that the game will allow the player considerable freedom to complete a mission, offering different ways to get through a challenge.
"You have to allow the player to author the experience, while giving them enough of a story to keep them pushing forward. We like to think of it as casting you as the star of your own action movie."
As part of this, the developers have devoted much time and effort to improving the facial expressions of some of the supporting cast as they wanted a player to feel a connection with characters in the game.
The other big change is the world that Valve has created.
It has come up with a highly realistic physics engine that lets you use anything in your environment, barrels, crates or logs, to defeat your enemy.
Then and now: Facial detail is much better in Half-Life 2
"We wanted to create a real setting, a world that simulates physically what happens in the real world," said Mr Lombardi. "It provides a really different environment."
In a demo of the game, wood floats in water while metal barrels sink and wrecked cars inch backwards when they are struck by bullets.
Valve has sought to reassure gamers that they will not need to buy the latest high spec computer to play the game.
It is aiming for a game that will run on a machine with a 733Mhz processor, with 128Mb RAM and a DirectX 6 graphics card.
And fans of the online shooter Counter-Strike will be pleased to hear that the new game will include a multiplayer option.
Up to 32 people will be able to battle each other in a variety of games, though few Valve is not giving any more details at this stage.
At the recent computer games show in London, Mr Lombardi insisted the game would be ready to ship on 30 September.
Half-Life 2 attracted much attention at London's recent games show
But since then there have been rumours that it may be delayed until November.
When it is released, Valve is confident that Half-Life 2 will go on to sell millions as the original did, based on the positive reaction to the snippets of the game it has shown to avid fans.
But this time round, it will face competition from other sci-fi shooters, such as Doom III and Halo 2.
"It is getting harder and harder to break new ground and not just pump out more of the same," admitted Mr Lombardi.
"We were worried if people would still care about Half-Life, but we are not worried about it any more."