Shadow Complex is Super Metroid for the Gears of War generation. Its 2D action mechanics are pretty much lifted straight out of the 1994 Nintendo classic, but it's unequivocally modern with production values breaching the stratosphere and spiralling off into space.

Created by Epic Games' Chair Entertainment, Shadow Complex is, in fact, a huge underground base, home to the Progressive Restoration, a fanatical terrorist organisation hell-bent on "liberating" San Francisco. You tumble into its innards accidentally. Jason Fleming (voiced by Nolan North, aka Nathan Drake) is enjoying the great outdoors with his girlfriend when she is captured by stormtrooper-esque bad guys. Armed only with a flashlight, Jason heads deep underground, braving flooded caves, monstrous mechs and an endless army of gun-toting goons as he desperately tries to rescue his princess and unravel the motivations behind the mysterious organisation's nefarious plans.

Shadow Complex's mechanics are old-school 2D in the extreme, but the game world is rendered in 3D and the animations are super slick. As is current de rigueur, a 2.5D approach has been taken, the gorgeous Unreal Engine-powered graphics providing some stunning backgrounds both over ground and underground, with eye-catching explosions rocking enemies and crates in all dimensions.

You're able to shoot left and right and up and down and anywhere in between, but you're also able to shoot into the background at enemies who appear in the distance. This takes some getting used to at first. Jason's movement is governed by the left thumb stick, but his weapons are aimed more precisely with the right thumb stick. Think Geometry Wars - you're able to move left while shooting right, jump up while shooting down.

Shadow Complex is a huge underground base full of mechs and enemy soldiers

Jason's agile. Really, really, agile. He's able to leap, wall jump and pull himself up ledges like a man-sized fly - all this with only the game's first discoverable item: his girlfriend's climbing gear. Item gathering proves to be Shadow Complex's main hook - the deeper down the rabbit hole you explore the more power-ups you discover and the more previously inaccessible areas become accessible.

Normal guns, for example, destroy orange grates. When you get grenades, however, you're able to destroy green grates and rocks. Red doors and rock surfaces can be destroyed with missiles, and purple objects, like ventilation fans, can be destroyed with the Foam Gun (a hugely fun weapon that shoots small blocs of foam that expand to form platforms or stick enemies in place).

It's not just Jason's weaponry that expands - early on he finds a piece of an advanced suit of armour that eventually leads him to transform into a super-powered cyborg. At first the upgrades are slight - thrust boosts for a double boost jump and damage reduction come in very handy, but then even more powerful armour pieces are found - breathing apparatus for underwater diving and the hook shot for Spider-Man-style swinging to name only two. The Friction Dampener, which enables you to run at hyper speed, lets you bulldozer your way through blue obstacles. By the end of the game you're motivated more by hunting down every single piece of armour than you are to save your poor old better half. The pursuit of 100 per cent completion is paramount.

Eventually, as you find new bits of armour, Jason becomes something of a cyborg ninja.

This is where the comparisons with Super Metroid are most applicable. Each new item found leads to new possibilities. You find yourself scouring the grid-like map for unexplored areas, previously impassable structures and rooms with question marks - legend for unfound item. There's loads of backtracking - a hallmark of the Metroid series - but it never feels frustrating because new goodies are always just around the corner. Shadow Complex is in a hurry: new weapons and armour upgrades come thick and fast - evidence, perhaps, of Chair's mindfulness of the modern day gamer's attention span.

Underpinning the item-hunt gameplay is an experience point-based levelling up system that quietly gets on with its business without ever making the player a slave to it. You'll level up almost without noticing, improving your accuracy, health and armour along the way. But if you bother to get your hands dirty with the system, you'll find a meta-game hidden underneath. Spectacular kills score experience gain multipliers. Kill a bunch of goons with a falling shipping crate, for example, and you'll get a huge amount of XP. String melee kills (press B up close) and headshots together for more bonuses. Kill a boss (the boss fights are great) by way of its weak points and you'll be rewarded appropriately. It's not just the doing, but the way of doing, that counts.

Critics will say Shadow Complex is just a rip-off of a genre once dead, but they're missing the point. It's executed so perfectly that accusations of plagiarism are easily forgiven. At a time when so many games feel like little more than re-skinned versions of Gears of War or Halo, Shadow Complex's somewhat archaic mechanics are like a breath of fresh air.

Critics will also point to the brainless Hollywood blockbuster plot, but they're also wide of the mark. Shadow Complex is perfectly pitched. It's based on Empire, the Orson Scott Card novel, which might be improbable and unrealistically gung-ho, but it fits. When Jason talks to his girlfriend or himself, or eavesdrops on enemy chatter, it comes off as wonderful attention to detail on Chair's part, not clichéd dialogue.

Shadow Complex is a game every 360 owner should play. Brilliant.

Chair's greatest triumph, though, is that it's nailed the platforming. It feels just right - only rarely frustrating. The collision detection is spot on - absolutely crucial for precision platforming, and the controls are as tight as a drum. The animations are gorgeous - from the holstering of weapons to the clambering up of walls - every action flows together seamlessly.

For a downloadable game, Shadow Complex is packed to bursting point. The main campaign, on the normal difficulty, won't take you more than five or so hours to complete, but that's only half the adventure. You're compelled to reload and hunt down every single item, explore every nook and cranny of the massive complex in search of that final piece of armour. And when you're done with that, there's a raft of time-based one-off challenges to pit your skills against. Online leaderboards will only fuel high score obsessions.

In short, Shadow Complex is a triumph, a new standard for downloadable games. It's a game in the purest sense, but is as "next-gen" as any Xbox 360 owner could hope for. And at 1200 MS Points it's an absolute bargain guaranteed to fill rival XBLA developers with fear. 2009 has already seen a raft of quality downloadable games, but Shadow Complex could well be the best.