Some gamers will argue that the "Halo" series of first-person shooters has been milked to death.

I admit I wasn't that excited to hear about "Halo 3: ODST," which technically is not a new game; it's more of a stand-alone expansion.

Then I played it. And I loved it.

"ODST" is a bit different from previous games, largely because you're playing as the titular Orbital Drop Shock Troopers.

That's right, there is no Master Chief, just some above-average soldiers. They're not as strong, they can't jump as high and they can't magically replenish their health once their shields are taken down.

Yes, this means you must depend on health packs.

"ODST" takes place parallel to the events in "Halo 2." The ODSTs are dropping to Earth to fight the Covenant, an alien race invading the planet, when a blast scatters them.

The story is told in an interesting manner. You start out as Rookie, who wakes up to discover he's all alone. He must search the city of New Mombasa for clues as to the whereabouts of his squadmates. These "clues" lead to flashbacks showing you what happened.

Except you don't just see the flashbacks, you play them. The game alternates between Rookie's quiet isolation and his squadmates' intense firefights.

The rather short campaign can be played cooperatively with as many as four players over Xbox Live. It's a little odd because the story doesn't account for any people other than Rookie, but I'm willing to overlook the incongruity because I love co-op.

Then there's "ODST's" new mode: firefight. This is a cooperative mode in which your team fights off waves of enemies until all your lives are gone.

Firefight is different from similar modes in other games because each round activates a set of skulls, which creates new obstacles for players. For example, you eventually see the skulls that give enemies twice as much health and the ability to deflect bullets.

Firefight is so much fun that, before you know it, it's 3 a.m. and your hands are cramping up.

"ODST" may not be a full-fledged new game, but the campaign is entertaining and the new firefight mode is absolutely fantastic.

"Halo" and shooter fans alike should give it a try.

Etc., etc.

-- Map happy: "ODST" is a great purchase for players who like playing "Halo 3" online but haven't downloaded any of the extra maps. "ODST" includes all of the maps, plus a few more.

-- Shiny: Joss Whedon fans will love the voice acting of the ODST squad. It includes "Firefly's" Nathan Fillion, Adam Baldwin and Alan Tudyk. Plus, there's outsider Nolan North. My dream team.

Aimee Green writes video game reviews for the Lincoln (Neb.) Journal Star, a Lee Enterprises newspaper.