The last time we saw Metal Slug on PSP was the tremendously entertaining Metal Slug Anthology, which gathered together a decade's worth of gaming action. Metal Slug XX is like the revival games we've seen from a few franchises like Mega Man, which takes the old formula out for a new spin. The modern-day gaming equivalent of a reunion tour for aging rockers, Metal Slug XX is worth every dollar to fans of the franchise. If you haven't played the classic titles that led up to this, Anthology is a good primer, but not "required reading" before you can enjoy Metal Slug XX. Everything about Metal Slug XX is retro, including classic characters that have been part of the action since Day One. A couple of new friends join the party, drafted from King of Fighters, but your favorites are all present and accounted for. The surroundings are varied, from underground to metro to just plain wild. Boss designs have never been better, and enemy actions are full of explosive graphics and sound effects.

The music is stirring, but nothing you'll be humming when you put down your PSP. The sound effects are awesome, including the groans of your enemies as you take them out with explosions and hunt them down from behind the protective armor of a commandeered 'Slug. Multiplayer action is fast and furious, without noticeable slowdown or lag. Level designs include branching paths that keep replays from feeling redundant, and the mix of side-scrolling and climbing also helps to make things feel fresher the second or third or fiftieth time... The Combat School mode gives you a series of bite-sized gameplay chunks that drop you into some part of a familiar level with a different objective each time. This creative use of the game's limited visual assets makes Metal Slug XX something you'll happily pick up and play far beyond the first time you see its credits roll.

If you're new to the Metal Slug world, get ready for some awesome side-scrolling, shoot-em-up action. The premise has been roughly the same for the past decade or so: You join a small elite bunch of soldiers intent on destroying Morden's forces. This time, there's a twist, as Morden is reinforced by advanced army forces that bear some unique weaponry. You'll now have the chance to wield lasers and lightning ray-guns, in addition to your usual armaments. You'll also have the chance to pilot some 'Slugs large and small, either in opposition to Morden's army or against enemy gun emplacements and bosses. Each level is compact but filled with challenges that only increase as you amp up the difficulty level. Enemies storm your position from both sides of the screen, and camp out behind heavy shielding. They lob bombs and fireballs at you, while you attempt to do the same to them. Each character in Metal Slug XX has a slight advantage that may suit your style of playing. Eri and Fio, the girls, both benefit from extra firepower; Eri packs extra grenades and Fio starts the game with a heavy machine-gun. Marco and Tarma benefit from upgraded standard weapons, while Ralf and Clark have awesome hand-to-hand combat skills.

Multiplayer requires that both players have a copy of the game, and are in close proximity to establish a wireless network. The payoff to this is the chance to play cooperatively through Metal Slug XX, which will bring you as close as you possibly can be these days to recreating the kind of arcade fun we used to enjoy. Other neat features include Combat School mode, which takes you through a series of special challenges set in the world of Metal Slug XX. While the settings are familiar, the objectives are different each time, and there are several levels of challenge. One time you'll be facing off against the bosses from each level, while the others may involve dodging projectiles or destroying a well-guarded facility against teeming enemy forces. Rankings and points for these Combat School missions mean you'll have plenty of reasons to keep coming back to Metal Slug XX. The standard goals for each level continue to be (in this order) staying alive, defeating enemy troops, rescuing POWs, and collecting special pick-ups for higher scores.

If you go out in a blaze of glory during the game, you'll have the opportunity to continue where you left off, but there are a finite number of continues before you'll have to restart the level. Playing on the easiest setting, you'll find Metal Slug XX accessible enough to complete in one sitting. This method doesn't really require a lot of skill, and the nuances of each character will probably be lost on you. As you become familiar with the enemies' patterns, you'll feel comfortable dialing up the difficulty setting, or exploring some of the game's alternate paths. Certain paths will prove to be more difficult than others, but the end-boss at each level is the great equalizer. Mastering the boss battles is a real challenge, and often depends on whether you've taken time to gather and save more powerful weapons. Rescuing POWs isn't a required part of the game, but helps with overcoming enemies since POWs are likely to drop a helpful weapon power-up. Saving weapons is always a bit of a gamble, since you'll forfeit them if you lose a life, but the ability to manage weapons strategically is a key piece of Metal Slug XX. Learning which characters match your playing style is also a strategy, but it will take a few rounds of play before you can appreciate the difference between them. The auto-fire setting is a nice way to simplify controls, but there's a lot to keep tabs on while you play.

The most basic elements of play during Metal Slug XX involve running with the Analog Stick, jumping with the (X) button, and attacking with the ([]) button. Beyond these simple controls, you'll find there are lots of variations on the simple attack. Closing in on enemies will render a hand-to-hand attack, still with the same button that fires your primary gun. There are jumping attacks that you can direct up or in front of you by tweaking the Analog Stick, or downward shots you can manage by pulling down on the stick, while jumping and shooting. The Analog Stick is very sensitive, which is a good thing once you get accustomed to it, but difficult to master until you've played through a few times. The more advanced controls are attached to techniques like lobbing grenades, or using special attacks that are only available to a few characters. Getting in a 'Slug will be familiar in terms of which buttons to press, but completely different in how the machines feel in action. Combat School sounds like training, but it really isn't anything of the sort. Metal Slug XX is a game that you'll have to be ready to jump into, with guns blazing, and learn as you go.

This is the essence of classic arcade gaming, recaptured for a new generation and perfectly suited to PSP. Taking the game with you opens up possibilities for co-op play with a friend, and the only thing better than playing with a friend is playing with three of four. This kind of Metal Slug mayhem isn't on the menu, but if Mario could pull it off, Metal Slug can't be far behind with four-player action. We certainly hope that there will be more of this good thing to come on PSP, as Metal Slug XX proves that classic side-scrolling action never gets old. It's a twitchy joint that rewards reflexes and almost nothing else, so it won't be right for every gamer. For those of us that cut our teeth on Metal Slug and other fast-action arcade action games growing up in the golden age of videogames, Metal Slug XX is like a "Back to the Future" moment where we get to relive some great times that actually never happened. About the only thing we wish this had was the ability to play true online multiplayer, or the option to join more players into a local wireless game. Otherwise, it's retro perfection.