In the stylized, cartoony "Crackdown 2," you play as a super-powered Agent who does battle with zombies and militants.

While “Crackdown 2” isn’t the first video game to make its players feel like superheroes, it might be the first one to nail what I’ll call the Bat Signal effect.

Plenty of superhero video games ask players to aid damsels in distress, save citizens from crumbling buildings or vanquish high-powered evil-doers. But “Crackdown 2” is the first game that had me bounding over rooftops, speeding in a car across a massive city or blindly wiping out everyone on an entire city block to bail out a real-life buddy who desperately needed my aid. It sure beats carefully making my way to a programmer-created amalgam of pixels and voice acting that waits around forever until I show up and trigger a scripted event.

Yet as pitch-perfect as “Crackdown 2” is with three or four players on Xbox Live, it can be a tedium to play solo.

Like its predecessor, “Crackdown 2” (rated M, $60 on Xbox 360) is an open-world action game that takes place in the fictional Pacific City. Once again, you play an employee of the cryptic Agency, a souped-up law enforcement body with hidden, sinister motives. As you play, your initially vanilla Agent will enhance his core skills — agility, strength, firearms, explosives and driving — by using them, until you’re leaping off of skyscrapers, throwing big rigs and setting off chain-reaction explosions the size of football fields.

But unlike the first game, which tasked one or two players with trying to take down three distinctly different gangs, “Crackdown 2” has a streamlined, simpler plot. To deal with an infestation of “Freaks,” essentially zombies who are sensitive to ultraviolet light, you have to activate nine UV-emitting beacons.

To prepare each beacon, you’ll switch on three different rooftop power sources called “activation units.” Working to stop you is a group called the Cell, an organization the agency refers to as terrorists, but whose motives are revealed to be more humanitarian than your sadistic, often hilarious boss leads you to believe.

The problem with “Crackdown 2,” however, is that it never fully explores its potentially rich world. Over the course of playing both “Crackdown” games, it’s easy to get the sense of a larger, more interesting narrative just waiting to be fleshed out. But the barebones narrative of “Crackdown” is replaced with a fossilized one in the sequel. Developer Ruffian Games, taking over from Realtime Worlds, has created a huge arena and built a heck of a game engine, but Pacific City has none of the character of “Grand Theft Auto IV’s” Liberty City or “Brutal Legend’s” heavy-metal-inspired landscape.

It’s the same story with the gameplay. You’re a superherolike Agent in an enormous metropolis where you can do anything you want, as long as the only things you want to do are hunt for “orbs” that power up your Agent, trigger activation units, defend beacons, capture Cell-controlled tactical locations and wipe out pockets of Freaks. (Killing Freaks and seizing Cell territory are essentially the same activity, just with different enemies.)

Oh, sure, there are the customary foot- and vehicle races, as well as a number of stunt jumps and gliding games, but these kinds of activities are staples of the genre and barely even count.

Played solo, the whole affair devolves into a tedious cycle of lather, rinse, repeat. As with the first game, collecting agility orbs placed high on rooftops so that you can jump even higher, run even faster and reach new orbs is an obsessive-compulsive good time. But you can’t hang a whole game on it.

I haven't unlocked helicopters yet, but I can't wait until I do.

Yet somehow, “Crackdown 2’s” online-only co-op mode covers its numerous warts and transcends the game’s limitations. While there are only a handful of official objectives, what is there is made more fun with each player you add to the mix. During the week I spent playing the title for review, I played solo, as well as with one, two and three other Agents. While the game was more fun with two players, it hit its sweet spot when three or four Agents were running around, tag-teaming the missions, jointly hunting for orbs and experimenting on the game’s large canvas. (You can also take part in online deathmatch, team deathmatch and rocket tag games, which support up to 16 players.)

Inevitably, at some point someone will wander off to chase an orb or complete a stunt race, then blunder into a hornet’s nest of rocket-launcher packing Cell. When the call for help goes out and triggers that virtual Bat Signal effect, it’s like nothing else in gaming.

If you’re part of a semiregular trio or foursome of gamers who’ve enjoyed such online co-operative titles as “Borderlands,” “Left 4 Dead 2” or Firefight mode in “Halo 3: ODST,” “Crackdown 2” is a logical next purchase. But if you play solo, or long for multiplayer games with split-screen, local co-op play, it’s probably not for you.