NEW YORK (CNN/Money) - When it was released in November 1998, "Half-Life" redefined the action gaming genre by combining fast-paced action and an intelligent, intense story. Players quickly began clamoring for a sequel, but developer Valve Software responded with silence. Five years later, the call for that sequel is as loud as ever.

Later this year, it will finally be answered.

Eli and Alyx Vance are two of your key allies.

"Half-Life 2" is scheduled to hit store shelves Sept. 30. Developed once again by Valve and published again by Vivendi's (V: Research, Estimates) Sierra games division, the game will put players back in the familiar shoes of crowbar wielding hero Gordon Freeman, battling aliens from the planet Xen.

For those of you scratching your head and wondering what the big deal is over a video game sequel, here are a few numbers to take into consideration. Since its 1998 launch, "Half-Life" and its various add-on packs (including mods such as "Counter-Strike" – see below for more on those) have sold more than 8 million copies worldwide, according to Valve. With zero fanfare, "Half Life 2" has tied (and possibly surpassed) "Doom III" as the most anticipated PC action game of the year – and could help spark a revival of the PC gaming industry.

Gordon Freeman takes up the crowbar again this September.

The game will make its public debut next month at the Electronic Entertainment Expo (E3), the trade show of the gaming industry, at the booth of graphic chip developer ATI. A non-interactive theater will show gameplay (much like id Software and Activision (ATVI: Research, Estimates) debuted "Doom III" last year).

"You are once again Gordon Freeman," described Gabe Newell, managing director and co-founder of Valve. "You are awakened in the future. Several of the characters from [the original game] are back, including Barney the security guard, the G-Man, about whom you learn more and in whose employ you now are, and several of the scientists. There are new characters as well, most prominently Alyx, whose mother was killed during the events of the original game. The story is once again conveyed entirely within the course of playing the game."

The game will also have multiplayer features, but Valve is holding those details back until it gets closer to shipping the game.

PC games have seen sales decline over the last few years, as consoles stole the spotlight. 2003 might rectify that with "Half-Life 2," "Doom III" and other, unannounced titles that will be unveiled at E3 next month.

"Holiday of 03 is going to be a really big year for the PC," said P. J. McNealy, research director at GartnerG2.

It's not just high-profile games that are behind that surge. The industry is nearing the apex of the console cycle and will soon start to see declines there. Typically, that's when PC titles pick up steam. PC hardware is now considerably faster than consoles – and the graphics capabilities of computers are noticeably superior.

While Alyx's father survived the first game, her mother wasn't so lucky.

As a result, said McNealy, "they need more intensive games – bigger, badder, faster. ... So far, there have been very few games that have challenged the latest PC builds. Graphically, there have been no challenges. We expect that to change this year."

The appeal of "Half-Life" goes beyond the game itself, though. Like many other action titles, "Half-Life" offered its basic programming code to whoever wanted it. Avid gamers use this code to make modifications – or "mods" – to the game, usually resulting in freely downloaded new levels or characters.

"Half-Life" fans, though, used the code to make entirely new games – called Total Conversions – which are freely distributed over the Internet. "Counter-Strike", the most popular of these, currently boasts over 1.7 million players per month, dedicating more than 2.4 billion minutes per month to the game, according to an October 2002 Business 2.0 story. Other total conversions, such as "Team Fortress Classic" and "Day of Defeat" are also exceedingly popular.

In order to play these conversions, though, gamers have to own the original "Half-Life", which keeps the money flowing in to Valve and Sierra. And Valve has bought the rights to the three conversions mentioned above. You can still download them for free, but Valve also sells shrink-wrapped copies at retail (or is working on retail versions), which have boasted remarkable sales figures. In fact, until last year, games in the "Half-Life" family had seen sales increase each year. Most game titles fade into obscurity within six months of their release.

With "Half-Life 2", Valve will once again offer the source code to modders.

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"We've learned a lot from our experiences with the mod community and with Counter-Strike and Day of Defeat," said Newell. "Half-Life 2 will be a much better platform for mod authors than Half-Life 1."

One of Half-Life's most popular mods was "Team Fortress Classic". Valve was so impressed with the game, it bought the rights and began working on "Team Fortress 2". But there haven't been any updates on the game since 1999.

There still aren't. But Newell did say "After we finish the announcement process for Half-Life 2, we will provide an update on Team Fortress 2."

The expectations among gamers for "Half-Life 2" are incredibly high – which might explain why Valve has kept its corporate mouth shut – even though it has been working on the game since it shipped the first "Half-Life" in 1998. If there's one lesson other developers have learned the hard way, it's that too much hype can crush a game before it even launches.  

Morris is Director of Content Development for CNN/Money. Click here to send him an email