The "Star Wars" movies should have taught all science-fiction writers a lesson: One trilogy is enough.

Bungie, the developer of Microsoft's blockbuster "Halo" trilogy, wrapped up the story in 2007. The war between humanity and the alien Covenant was over, with the hero, supersoldier Master Chief, drifting off into cryogenic sleep.

But while Master Chief's role may be finished, there are probably many more side stories to be told about the interstellar war. "Halo 3: ODST" (Microsoft, for the Xbox 360, $59.99) is one of those stories -- and your enjoyment of it will depend on how devoted a "Halo" fan you are.

"ODST" follows a squad of grunts dealing with events touched upon in "Halo 2." These Orbital Drop Shock Troopers come to Earth during an alien invasion of the African metropolis New Mombasa -- but their mission quickly goes awry and they're scattered across the city.

One trooper, "the Rookie," wakes up six hours later and sets out to find his five squadmates in the city's abandoned nighttime streets. Each time he finds a device connected to another soldier, the action switches to a flashback covering that soldier's role in the battle of New Mombasa.

It's a clever device, letting you experience different types of combat in a short time. In one scenario, you're asked to defend a rooftop stronghold against wave after exhausting wave of alien fighters. In others, you're asked to steer a Warthog all-terrain vehicle or a Banshee fighter plane through scenes of explosive mayhem. Each of the battles is presented with Bungie's usual expertise at making things go boom on a massive scale.

Still, none of these experiences feels fresh. And while it's fun to try on the combat boots of someone other than Master Chief, none of the new characters is more than a battlefield cliche. You can whip through the solo campaign in about six hours, and its climactic revelation is a letdown.

I was particularly disappointed in New Mombasa, which has to be the blandest city ever created, with none of the idiosyncrasies of a genuine Earth metropolis. Even after an alien invasion, it looks like a mall.

The Halo Nation thrives on multiplayer action, and "ODST" delivers a new cooperative mode called "Firefight," in which four players have to stand their ground against a flood of Covenant enemies. It also includes a disc full of multiplayer maps, including three new ones.

Looking at the "Halo" epic from another angle turned out to be much less interesting than I'd expected, and I couldn't shake the feeling that Bungie is bored with the franchise. "ODST" doesn't have the spark that made earlier "Halo" titles so compelling -- a spark that, I hope, will return when the studio moves onto something new. Two stars out of four.