Don't worry, I'm not going to spoil anything here - I'll steer clear of anything story-related beyond the premise. With another game, that would be tricky. With Skyrim, the stories that come from how the game works are often the best ones.

It's a frozen nation, just to the north of where the previous game, Oblivion, took place. A pleasantly brief introduction sets up the plot: Skyrim is in the middle of a revolt, you've been sentenced to death, and dragons have just shown up. Good luck!

At that point, you emerge from a cave into 40 square kilometres of cold and mountainous country, and that's it. Everything else is up to you.

Even after spending hundreds of hours in Morrowind and Oblivion, the sense of freedom in Skyrim is dizzying. The vast mountains in every direction make the landscape seem limitless, and even after exploring it for 55 hours, this world feels huge and unknown on a scale neither of the previous two games did.

Not all of the landscape is subzero, and even among the frosty climes there's an exciting variety: ice caverns that tinkle with dripping frost crystals, hulking mountains with curls of snow whipped up by the howling wind, coniferous forests in rocky river valleys.

The mountains change everything. Wherever you decide to head, your journey is split between scrambling up treacherous rocks and skidding down heart-stopping slopes. The landscape is a challenge, and travel becomes a game.

It's hard to walk for a minute in any direction without encountering an intriguing cave, a lonely shack, some strange stones, a wandering traveller, a haunted fort. These were sparse and quickly repetitive in Oblivion, but they're neither in Skyrim: it's teeming with fascinating places, all distinct. It was 40 hours before I blundered into a dungeon that looked like one I'd seen before, and even then what I was doing there was drastically different.

These places are the meat of Skyrim, and they're what makes it feel exciting to explore. You creep through them with your heart in your mouth, your only soundtrack the dull groan of the wind outside, to discover old legends, dead heroes, weird artefacts, dark gods, forgotten depths, underground waterfalls, lost ships, hideous insects and vicious traps. It's the best Indiana Jones game ever made.

The dragons don't show up until you do the first few steps of the game's main quest, so it's up to you whether you want them terrorising the world as you wander around. A world where you can crest a mountain to find a 40-foot flying lizard spitting jets of ice at the village below is a much more interesting one to be in. But fighting them never changes much: you can just ignore them until they land, then shoot them from a distance when they do.

Your first dragon kill is a profound, weird moment. I rushed to the crashed carcass to loot it, then looked up. The whole town had come out to stand around and stare at the body, a thing as vast and alien to them as a T-rex in a museum.

I tried shooting an ice bolt at it, just to demonstrate it was dead, and the force unexpectedly catapulted the whole thing violently into the distance. A beggar looked at me and said, "Oh sure, just throw your trash around."

Your character gets better at whatever you do: firing a bow, sneaking up on people, casting healing spells, mixing potions, swinging an axe. There's always been an element of this practice-based system in Elder Scrolls games, but in Skyrim it's unrestricted - you don't have to decide what you're going to focus on when you create your character, you can just let it develop organically.

That alone would feel a little too hands-off, but you also level up. When that happens, you get a perk point: something you can spend on a powerful improvement to a skill you particularly like. Every hour, you're making a major decision about your character's abilities.

They're dramatic. The first point you put into Destruction magic lets you stream jets of flame from your hands for twice as long as before. As you continue to invest in one skill, you can get more interesting tweaks: I now have an Archery perk that slows down time when I aim my bow, and one for the Sneak skill that lets me do a stealthy forward roll.

Again, the freedom is dizzying: every one of 18 skills has a tree of around 15 perks, and the range of heroes you could build is vast. I focused on Sneak to the point of absurdity - now I'm almost invisible, and I get a 3,000% damage bonus for backstabs with daggers. It's the play style I've always wanted in an RPG, but I've never been able to achieve it before.

The enemies you encounter are, in some cases, generated by the game to match the level of your character. In Oblivion that sometimes felt like treading water: progress was just a stat increase, and your enemies kept pace. That doesn't apply now that your character is defined more by his or her perks, because the way you play is always changing.

Levelled content is also just used less: at level 30, my most common enemies are still bandits with low-level weapons. And I still run into things too dangerous for me to tackle.

Taking a narrow mountain path to a quest, something stops me in my tracks: a dragon roar. I check the skies - nothing, but I hear it again three more times before the peak.

At the top I find a camp full of bodies, with a large black bear roaring over them. Hah. He's still more than I can handle in straight combat, but as he reaches me I use a Dragon Shout. It befriends any animal instantly, and he saunters casually away. Feeling slightly guilty, I stab him in the back before it wears off.

Which is when the dragon lands, with an almighty crash, six feet from my face.

I run.

A roar of frozen air catches me in the back, but I keep going - over a ridge, down a short drop, and straight into a bandit. I dodge the bandit, straight into a Flame Atronarch. There are five more bandits behind it. The dragon is airborne. I throw myself off the mountain, several hundred metres into the river below.

I plummet to the riverbed, and swim until I run out of breath. When I surface, the sky is alight with fireballs and flaming arrows, the dragon is spewing a stream of ice down on the bandits, and I'm laughing.

The stealthy character I built in Skyrim would have been less fun in Oblivion. Whether you were detected was a binary and erratic matter. Skyrim cleverly gives you an on-screen indication of how suspicious your enemies are, and where they are as they hunt for you. It makes stealth viable even against large groups: if you're rumbled, you can retreat and hide. And there's a slow, methodical pace to it - long minutes of tension broken by sudden rushes of gratification or panic.

Magic, meanwhile, has been given an incredible crackle of raw power. Emperor Palpatine would be a level one mage in Skyrim - unleashing two torrents of thrashing electrical arcs is literally the first trick you learn, and it doesn't even get you tossed into a reactor shaft.

One tweak is a huge loss, though: you can't design your own spells. Oblivion's spellmaking opened up so many clever possibilities - now you're mostly restricted to what you can buy in shops.

While we're on the negatives, physical combat hasn't improved much. There are cinematic kill moves when your enemy is low on health, but whether they trigger seems to be either random or dependent on whether the pre-canned animation fits into the space you're in. Too much of the time, you wave your weapon around and enemies barely react to the hits.

The exception is archery: bows are now deliciously powerful, and stealth shots can skewer people in one supremely satisfying thwunk.

What does improve the general combat is a feature I didn't quite expect: you can hire or befriend permanent companions. I did a minor favour for an elf at the start of the game that earned me his loyalty for the next 40 hours of play. Sidekicks add a wild side to fights: an arrow from nowhere can end a climactic battle, or a misplaced Dragon Shout can accidentally knock your friend into an abyss.

The Dragon Shouts, gained by exploration and killing dragons, are like a manlier version of conventional magic. One can send even a Giant flying, one lets you breathe fire, another makes you completely invincible for a few seconds. Even the one for befriending furry animals is macho: it can turn four bears and a wolf pack into obedient pets with one angry roar.

Before I got the animal shout, I had a Sabre Tooth problem. Crossing a fast-flowing river at the top of a waterfall, a huge feral cat spotted me. A good shot with a bow made no dent on its vast health bar, and it splashed into the water to get to me. The current was too strong to get away in time, so I did the one thing it couldn't: turned invincible and threw myself off the waterfall.

After seconds of freefall, I hit the rocks, got my bearings, and looked up. The cat - a speck above - seemed to be looking over the falls at me. Then it slipped. Its lanky ragdoll smacked every rocky outcropping on the way down, and wedged between two stones directly above me, his huge head glaring emptily.

The first few quests you're nudged towards get you the Dragon Shouts. After that, the main quest is a bizarre mix of some of the best moments in the game, and some of the worst.

It fails where the previous games fail: it tries to make your mission feel epic by making it about a prophecy, then does all its exposition in the time-honoured format of old men giving you interminable lectures. The acting is stagey at best, painful at worst. And it adds a new problem: your dialogue choices are now written out in full, and your only options are to react like an incredulous schoolchild to every predictable development. It doesn't make it easy to feel like a hero.

The main quests themselves are mostly good: a happy mix of secrecy, adventure, and exploring incredible new places. One location, which I won't spoil, got an actual gasp. But then there's an abysmal stealth mission that seems to work on a logic entirely its own: guards spot you from miles away, despite facing the wrong direction. And the boss dragons it keeps throwing at you never get any more interesting to fight - adding more hitpoints just makes the repetition even harder to ignore.

Everywhere else, the quests are magnificent. Chance encounters lead to sprawling epics that take you to breathtaking locations, uncover old secrets, and pull interesting twists. Even the faction quests are better here. It feels like Bethesda realised these became the main quest for many players, and built on that for Skyrim. They start small, but each one unravels into a larger story with higher stakes. Some of them feel like the personal epic that the main quest has always failed to be.

We got a review copy of Skyrim the day the game was officially finished, but it's curiously buggy. Among a lot of minor problems such as issues reassigning controls, there's glitchy character behaviour that can break quests, and AI flipouts that can turn a whole town against you. And the interface isn't well adapted to PC: it sometimes ignores the position of your cursor in menus. There's an update due as soon as the game's out, but there's a hell of a lot to patch here. Next time, maybe don't commit to a specific release day just because it has a lot of elevens in it?

These aren't engine issues, though. Skyrim is based on tech Bethesda built specially for it, rather than the middleware engine used by Oblivion and Fallout 3. It's a lean, swift, beautiful thing. New lighting techniques and a fluffy sort of frozen fog give the world a cold sparkle, and the previously puffy faces are sharp, mean and defined. Even load times are excitingly quick. On maximum settings, it runs at 30-40 frames per second on a PC that runs Oblivion at 50-60 - a decent trade off for the increase in scenery porn.

There's a lot of that. There's a lot of everything, and you have totally free rein of it. Skyrim feels twice the size of Oblivion, despite being the same acreage, just because there's so much more to see and do. Searching for Dragon Shouts is a game in itself. Exploring every dungeon is a game in itself. Each one of the six factions is a game in itself. So the fact that the main quest is a mixed bag doesn't hurt Skyrim's huge stock of amazing experiences.

The games we normally call open worlds - the locked off cities and level-restricted grinding grounds - don't compare to this. While everyone else is faffing around with how to control and restrict the player, Bethesda just put a fucking country in a box. It's the best open world game I've ever played, the most liberating RPG I've ever played, and one of my favourite places in this or any other world.

In case I'm not getting it across, this is a thumbs-up.