After Master Chief's last battle with the aliens who tried to destroy Earth, he vowed to finish the fight.

Three years later, hard-core fans are gearing up for his return with Tuesday's release of "Halo 3", the final chapter of the most popular video game franchise ever.

"I'm on the computer every day at college looking for news and updates," said Kevin McGoran, 18, a freshman accounting major at Community College of Allegheny County. He said he has taken a week off from his job at Panera Bread just to play the new game. "I'll be ordering pizza the whole time."

Halo is far more than a hot video game. It's a worldwide obsession that has spawned a multimillion dollar marketing campaign on a scale some have compared to the release of the "Star Wars" series.

Mountain Dew is promoting "Halo 3" with a caffeine-loaded, 20-ounce soft drink called Game Fuel, which has a wrapper depicting Master Chief in scenes from the game. Other marketing partners lined up by Microsoft for "Halo 3's" launch include Burger King, Pontiac, Doritos and Comcast.

Its predecessor, "Halo 2," racked up $125 million in sales within 24 hours of its release in 2004. Microsoft and Bungie Studios, the company that created the game, expect "Halo 3" to beat that and generate the highest single day of gross sales in entertainment history. The game can be played only on Microsoft's Xbox 360, whose sales are bound to profit from the new game.

"Obviously to these fans, Halo is more than just a game. This will be a chance for them to really buy into the franchise to own a piece of something they're passionate about," said Lora Cohn, vice president of Fox Licensing in Los Angeles, the primary licensing agent for Halo.

A storm of Halo-related products are set to roll out with the game's release, including T-shirts, wristbands, wallets, stickers, key chains, belts and bags.

"There will be products at the game launch, but it will continue through the beginning of next year and beyond," Ms. Cohn said. "From a licensing standpoint we look at this not just as a game launch, but as a franchise opportunity. It will have legs beyond 'Halo 3'."

Good, evil and more

As game story lines go, Halo's is pretty basic. It's the classic last human standing against the alien invaders scenario, but it's not just the cool weapons, massive landscapes and epic action that attract players to the Halo universe. The complex mythology, mixing military themes with themes of faith and spirituality, resonate with real-world issues. At its core, Halo is a nod to historical heroes of World War II and Thermopoli and sci-fi classics like "War of the Worlds," with Master Chief and his allies representing those fighting the good fight to save humanity.

"The tale of lone space marine fighting massive race and army of aliens that are hellbent on destroying planets in the universe and Earth, it's nothing new, right? We've seen that before," said Kevin Pereira, host of "Attack of the Show" on the G4 cable TV network. "But what Bungie does, the way that they weave that tale, the way that they tell it, with the quality of their cinematics, their sound and their immersive game play, it just sucks you in."

The game can be played alone or in multi-player combinations -- two against two or four against four if two Xboxes are linked together. With a really big room and four Xboxes, four teams of four can compete against each other.

Internet play really took off with Halo 2 and the growth of Xbox Live, which allowed players to connect the Xbox to the Internet and play games online against people in other areas.

Multi-player competition can be complex. Gamers use a ranking system to make games more fairly reflect their skill and experience. They can play not only as Master Chief, but also as a Covenant alien. Players can compete either to make the most kills or to capture the other side's flag.

"The typical amateur player would be a male teenager," said Leon Edelsack, owner of Pittsburgh CyberConXion in Squirrel Hill, which hosts Halo tournaments. "What appeals to men is shooting things up. But it's also a social thing with camaraderie and competition."

Halo 2 also sparked the trend of semi-pro and professional players in the gaming industry, with corporate sponsors were lining up to support leagues the players established, such as Major League Gaming and World Cyber Games.

"This audience has been around for a while. Kids have been wanting this kind of story and no other game creates as compelling a universe as Halo," said Jonathan Drubner, a video game expert and host of Life-Skool's video-on-demand series Gamer Dojo.

"The question of the phenomenon driving Halo can be answered in a 100-page term paper or in two sentences," Mr. Drubner said. "Games should be fun, cool and interesting."

A boost for Xbox

Just as the original Halo pushed sales of the Xbox, "Halo 3" promises to do the same.

"Halo is really a system seller. It's a breakthrough game for Microsoft," said Jeannie Novak, online program director for the Art Institute of Pittsburgh. "It does have that mythic quality. Because the early games are so great it attracted a cult following."

Big game releases typically are held in late October or November in preparation for the holiday season. Some industry observers think Microsoft is releasing "Halo 3" early to boost sales of Xbox 360 and blunt sales of Sony's Playstation 3.

"Sony keeps themselves up at night trying to find the Halo killer," Mr. Drubner said. "It's a nightmare for them."

PlayStation 3 competes with Microsoft's Xbox 360 and Nintendo's Wii as part of the seventh generation of video game consoles. In sales and market share, it's currently in third place. Xbox is No. 1.

"I know people who come here to play that say they are buying Xbox 360 because of 'Halo 3'," Mr. Edelsack said of his Squirrel Hill store. "I know people who actually have Playstation 3 and because they don't see anything they want to play are selling their Playstation 3 to buy Xbox 360 to play 'Halo 3'."

Bungie has promised Halo fans that "Halo 3" will finish the fight Master Chief started and has sworn there will be no other games in the Halo series.

Fans like Jake Meyer believe this is definitely the last Halo and that is why he has pre-ordered the Legendary edition of the game, which is the most expensive version at $129.99. It also comes with a replica of Master Chief 's helmet with its solid gold reflective visor. The standard edition costs $59.99. The limited edition is $69.99.

"The wait is kind of nerve-racking," the 15-year-old from Regent Square said. "You don't even know what it's going to be like, if it's going to be good. But I trust [Bungie]. I hope I'm going to get my money's worth."

Kevin Frankola, 15, of Wilkins got a small sample of "Halo 3" by downloading the multi-player beta version that Microsoft released in June.

"That held me off and decreased my stress in waiting for it," the Central Catholic High School student said. "And it made me really, really psyched for the game. It's really hard to wait for it."

By late evening tomorrow, people are expected to have formed miles of lines snaking along sidewalks and streets outside Best Buys, EB Games and Wal-mart stores across the country in anticipation of the midnight release. Plenty of them probably even will be dressed as Master Chief.