In theory, there shouldn’t be a ten out of ten score sitting at the top of this review. In theory, Mario should have run out of steam long ago. Just take a look at the competition. Look there, at Sonic, the ageing hedgehog has-been, every year promising a comeback, every year delivering another miserable dose of disappointment.

Look at Mario’s other would-be rivals - MegaMan, Busby, Gex, Crash Bandicoot, Blinx, Spyro the Dragon - they’ve had their moments, but the glory years are definitely over.

Sure, Ratchet and Clank still put in some decent competition, but they're 20 years younger than the big M, and only nine games into their career. Somehow Mario keeps on pulling out a blinder. How on Earth does Nintendo pull it off?

Simple. Nintendo has carefully and brilliantly evolved a set of brilliant game mechanics across five generations of console, and seems to have a well of ideas that never runs dry.

Even the series' biggest misfire - Super Mario Sunshine - had its share of ingenious little thoughts. And while Super Mario Galaxy 2 is a sequel, and so inevitably shares a core with the first Super Mario Galaxy, there's no sign that well is even close to empty. In fact, SMG2 throws out new Mario costumes, new worlds, new creatures and new devices with something approaching a manic intensity. That's just one thing that makes it a continual joy to play.

We won't go into details of the plot. Needless to say it involves Princess Peach, kidnapping and key members of the Bowser family, along with a hoard of super-power stars stolen by the aforementioned villainous turtle-shelled chaps.

With the aid of a planetoid space craft and its oddball crew, it's up to Mario to scour new galaxies, collect more power stars, and take the ship to the fringes of the universe to bring Princess Peach back home. It might be set in space, but Mass Effect this is not.

If you played the original Super Mario Galaxy, then you won't find the basic principles that much different. This is a 3D platformer with a difference. Each level, or galaxy, links a selection of tiny planetoids, some flat, some round, some shaped like winding roads, islands, strange machines or rocky mountains, via a system of paths and startrails, the latter propelling a flying Mario from one planetoid to another.

The weird, brain-bending 360 degree gravity returns, with Mario being able to run right around some planetoids or transit through pipes from one side to another, and once again Nintendo uses this stuff to the full to test the old grey matter. 

As well as the usual jumping over chasms and hopping on enemies to destroy them - along with the remote-shaking spin attack unveiled in the first SMG game - you'll come across a range of other tasks, items, mechanisms, puzzles and challenges. It's in these that the real joy of the game resides. 

To be honest, we don't really want to tell you anything about what goes on, because so much of the fun is in discovering SMG2 for yourself. We will mention that, as well as the old Fire Mario costume and the Bee costume from SMG, you'll get your hands on a mushroom that transforms you into a rolling chunk of granite and a drill that punches through from one side of a planet to another.

We'll also tell you that Yoshi returns to the series, with some superbly executed ways of munching and spitting out enemies at targets, a fabulous chilli-powered uphill sprint and a great tongue-swinging manouvre.

The game's genius isn't just in these tools, but in the way the game has you use them, always keeping the controls themselves simple and responsive, but throwing in little tweaks and layers of complexity that keep you exploring and discovering all the time. You’d have to be a particularly miserable sourpuss not to grin from ear to ear.

Along the way the game messes with gravity, physics and destructible objects in all sorts of ingenious and thrilling ways, and pulls you along, always pushing your skills, at exactly the right pace. Despite the fact that SMG2 is a noticeably more difficult game than SMG1 - even in the early stages - it never feels punitive or unfair.

Extra lives are doled out generously, and when you get really stuck, the game borrows New Super Mario Bros Wii's idea of an autopilot feature that will crack it for you, but without giving you the full reward. You can make progress again, but you'll always feel like you'll have to go back and crack the level for yourself. Most importantly, the controls are near-enough flawless. Given how much concern there was to the introduction of motion-controlled moves in SMG1, this is a miracle in itself.

The galaxies themselves are awesome creations. It's as if making a sequel has given Nintendo added confidence to push the player further, and create levels that look confusing or utterly bewildering at first, but actually make perfect sense as you play. There are secrets to discover and puzzles to solve, but it's rare to find yourself stuck with no idea of what to do, and you'll find helpful hint screens dotted around that can show you a quick hint video of how you might proceed.

This is also a big game, packed with hidden challenges and secret planets. You will occasionally find yourself forced to return to an old galaxy to gather an additional star, but when that happens there's still a sense of discovery. Each time you return, you’ll find something new you didn’t find before. 

If the gameplay is peerless for the genre, then SMG2 takes over from SMG1 as the most impressive demonstration yet of what the Wii can do when pushed. The more we get used to shoddily-built Wii games with simplistic cartoon graphics or ported PS2 models and textures, the more we start thinking that Nintendo's machine can't produce exciting visuals. SMG2 makes you realise how wrong that assumption is.

Sure, it sticks to bright colours and simple textures and lets the lighting and material effects do the heavy lifting, but we'd defy you to look at some of the galaxies on offer here and say that they don't look stunning. And when the game throws on the spectacle, as it does in the big boss battles, it's genuinely epic stuff.

Put SMG2 up against the last two Ratchet and Clanks and - barring the difference in screen resolution - it's hard to say that one is running on superior hardware. This is not to criticise Insomniac, just to say how incredible Nintendo's work here is.