NEW YORK -- They're not calling it a "Grand Theft Auto" expansion; they're calling it a whole new game.

Rockstar Games gave MTV News a hands-on demonstration with "Grand Theft Auto IV: The Lost and Damned" at the development studio's New York City headquarters yesterday.

"The Lost and Damned" is a $20 downloadable episode add-on to "GTA IV" -- coming exclusively to the Xbox 360 on February 17 -- that requires ownership of the "GTA IV" game disc.

How can they even suggest that it's a full game? Simple. Rockstar's vice president of product development, Jeronimo Barrera, told MTV News that the adventure could run some players as much as 20 hours and will have to be played over "several marathon sessions." It's not like other video game add-ons, he said. "This isn't a magic hole that opens up that's got some sh-- in it."

"GTA IV" protagonist Niko Bellic is just a supporting character in "Lost and Damned," with biker Johnny Klebitz taking the lead role -- Niko shows up as a bit player in a few of the episode's missions, which are repurposed from "GTA IV" and played from Johnny's angle. But most of the story and action in "Lost and Damned" is new. The episode focuses on the motorcycle gang the Lost, in which Johnny serves as the vice president. Johnny had been running the gang while its president, Billy, was in jail. Johnny made truces with rival gangs but, upon Billy's return, an older order is restored and Billy guns down at least one rival gang member.

Niko Bellic's story was about an immigrant and the American Dream. According to Barrera, Johnny Klebik's story is about "themes of loyalty and brotherhood that are just as interesting."

"Lost and Damned" is set in the same Liberty City as "GTA IV." The episode uses the graphics stored on the "GTA IV" disc to render the city, but the downloadable new game provides extra gloss and grit. The game's lighting has been re-worked to appear more dramatic, and the fidelity of character models has been improved. Handling of the game's motorcycles has been tweaked to make them less treacherous (though at press time that tweak was not set to affect handling of those bikes in the core "GTA IV" game). The added grit can be seen both in new menu overlays as well as the geographic focus of "Lost and Damned" -- the New Jersey-inspired sector of Alderney, which "GTA IV" players only visited in the core game's final fourth of its story line.

The game's story runs in parallel to that of "GTA IV," though Rockstar reps won't say if it goes further in time than Niko Bellic's did.

The game plays a little differently from "GTA IV." Players will have new weapons -- including a sawed-off shotgun -- and a new game-play mechanic that rewards players who ride Johnny's bike in formation with other members of the Lost with extra dialogue and health bonuses. There will be a hideout and friends, though no new girlfriends and no need to develop friendships to get special perks. A drop-down menu shows the biker allies' health, which seems to incentivize the player, letting the Lost gang enter missions with him and stay alive to maintain strength in numbers. (If they die, Barrera said, they are revived for the game's next mission.)

Rockstar will be offering new music and DJ chatter in the "The Lost and Damned" and new multiplayer options, but details on both are being kept quiet for now.

To access all of this content, players will pick "The Lost and Damned" from "GTA IV"'s main menu -- that's just another way of Rockstar expressing that this is a standalone experience.

"People might say that we're milking it," Barrera said. "But we're not. This is not a mod. These are not re-skinned characters doing delivery missions. We're rethinking how you play the game."

"The Lost and Damned" is a whole new "GTA IV" experience and a familiar city. All Xbox 360 players with $20 to burn can see how it plays when it is released on February 17.

A second "GTA IV" episode is also coming from Rockstar for the Xbox 360, but the company isn't talking about that one yet.

That's not all the "Grand Theft Auto" news we have for you today. Check out our blow-out on "Grand Theft Auto: Chinatown Wars," the series' first DS game, at our Multiplayer gaming blog.