It promises to be Canadian developer BioWare's best game to date. It promises an unrivalled cinematic experience. It promises to tug on your heart strings more violently than any interactive story in history. It promises many, wonderful things. Now, after a mammoth, unhindered two hour hands-on session beginning at the very beginning, Mass Effect 2 has finally stopped promising and started delivering.

What you are about to read is undoubtedly a spoiler. If you want your Mass Effect 2 experience to feel as fresh as possible, go read something else and, when you're done, give up the internet entirely. But what you are about to read is also just one account of a story that can be twisted and turned in as many directions as the player wants. Decisions were made that you may not make. Words were spoken that you may never speak. Words were heard that you may never hear. Whichever truth is your truth, you're about to go deeper underground than ever before...

Mass Effect 2 begins with a cutscene. The chain-smoking Illusive Man (voiced by Martin Sheen), head of the pro-human group Cerberus, is discussing the Reapers, Mass Effect's big bad evil alien bad guys, with brunette biotic Miranda (voiced and quite clearly modelled on Chuck starlet Yvonne Strahovski). Human colonies are being attacked and their populations kidnapped all across the galaxy. Cerberus knows the true nature of the situation, and the pressing need to do something about it. The answer: Commander Shepard is the only person for the job.

The game, without a Mass Effect save to analyse, assumes the Galactic Council was lost during the Reaper attack on the Citadel. Shepard and the rest of the crew, including the Seth Green-voiced Joker, Navigator Pressly, Doctor Chakwas and Kaidan Alenko, are hunting for Geth in the Terminus Systems. They detect something, something that isn't Geth. It's a giant ship, one that looks, somehow, as if it's built into a moon-sized rock. We see Shepard from the back as he or she puts a helmet on. Arguments ensue as Shepard forces the crew into escape pods. The mysterious ship fires and the Normandy's hull is torn apart. Now, the player is in control.

As you move through the ship towards Joker, Shepard shields his or her face from the burning flames. This is all you can do at this point: The Normandy is almost obliterated. At one point, you pass through a section that is now exposed to the cold, airless space that once surrounded it. Here, you can hear Shepard breathing calmly - not one, as we know, to panic under pressure.

Bam

Eventually you reach, and save, Joker, but at the expense of your own life. The Normandy's complete destruction approaches, and Shepard is thrown into space. As air escapes the suit, the Commander is seen spiralling towards an unnamed planet... This, of course, is what BioWare had us believe was the death of Commander Shepard. What stuff it was. His survival was as predictable as the release of this sequel. But, hey ho. Don't we just love hanging on tenterhooks?

Now, we get to the juicy part. Shepard did die. Well, sort of. We see the Commander's body being remade by robotic arms; it is an operation, part of something called "The Lazarus Project". Now, Mass Effect 2 begins. Do you quick start with the default male or female Commander Shepard (earthborn, sole survivor, soldier class), or do you create a custom Commander Shepard, tugging at Shepard's face like a puppeteer in charge of the robotic arms performing the facial reconstruction?

Custom, of course. Here, as was the case at the beginning of Mass Effect, you sculpt the face of the Reapers' demise. Multiple sliders are your scalpels. The randomise button is for those without the patience to indulge. Then, as before, you pick a class. This time, however, it's a "Cerberus class". Have you worked it out yet?

The six classes available are lifted verbatim from the first game. Today, though, it's the Infiltrator's turn to shine. Its powers raise the hair on the back of the neck: Tactical Cloak; Incinerate, AI Hacking, sniper rifle, heavy pistol, sub machine gun, cryo ammo, disruptor ammo... oh my. Then, as before, choose your background (spacer, colonist or earthborn) and psych profile (sole survivor, war hero, or ruthless). Accept, move on... next cutscene.

Part of Shepard's task is to keep his party loyal. Fail, and who knows what could happen?

Miranda is standing over you as you wake. Shepard has a scar on her (yes, this Infiltrator is a she) cheek, some kind of cybernetic implant, but the rest of her body is exactly as before, if that's what you, the player, wants, of course. Then, suddenly, there is an attack. Someone is trying to kill her. Shepard gets up, you gain control, and it's time for action.

Miranda guides Shepard's first tentative steps via voice communications. You grab a pistol, a thermal clip (ammo, essentially) and work through the tutorial. The game puffs out its chest: look at my new and improved cover system, marvel at my brilliant third-person shooting, gasp as you seamlessly trigger abilities and send enemies flying through the air with god knows what biotic ability. If Mass Effect 2 begins with an arrogant bang, it is a justified one: the "press A to get in, B to get out" cover system is the best I've experienced since Gears of War 2.

Hacked mechs are attacking Shepard. Why? Who knows? Now is not the time for questions. Now is the time for battle. You grab an N-100 Grenade Launcher, the first of the new heavy weapons, from the corpse of a security officer. A holographic image of Miranda, triggered by accessing a Cerberus Log, tells you Shepard is showing promise and the project has cost an eye-watering four billion credits. There are two new hacking mini-games, one involving matching two-of-a-kind symbols within a set time limit, the other requires you to match scrolling code with static code while avoiding bad code. More hacked mechs, more cover, more shooting.

You saddle up to Jacob Taylor, a human biotic who is fighting for his life from behind cover. You converse as enemy fire whizzes by your head; immediately you get a sense that talking with NPCs won't be as static an experience as it was in the first game. The superb conversation wheel again is the conduit through which you interact with the game's inhabitants. The mechs were your security, Jacob tells you. You've been out for two years. Two years! The Alliance officially declared you KIA. Bummer. You're not a clone. You are Shepard, but with extra bits and pieces. Pressly was killed, but everyone else survived.

You move on, now with Jacob as a party member. The power wheel triggers his special abilities, as well as yours, as before. Left to his own devices, he's pretty useful, shooting as necessary. You meet up with Wilson, a human crew member who's been shot in the leg. You use Unity, a healing power lifted from the first game, to revive him. Wilson joins your party, despite his dodgy personality and shady excuses. The game asks you to use his Overload power on some crates to take out a number of mechs at once. Boom. Miranda, finally, turns up, and shoots Wilson dead. She is director of the Lazarus Project, and wants you to escape on a shuttle and head to see the architect of your miraculous recovery: The Illusive Man. Thus ends Mass Effect 2's breathless beginning.

Mass Effect 2's first two hours see Shepard face off against waves of mechs

During the trip Miranda asks you questions (why did you leave Ashley to die?) on what happened during the first game. This is BioWare's sneaky way of allowing the player to change history, if indeed that's what you want to do, while carrying your character over. On The Illusive Man's pristine space station, Shepard walks around in casual clothes (there are a number of types). BioWare has separated the aesthetic from the statistical. You can change your appearance (material, tints, patterns and colours) at will, but your armour needs to be obtained. This time, though, instead of having one piece of armour, you've got five: helmet; chest; shoulders; arms and legs. In Mass Effect 2 there's more scope for customisation and more to get your fussing fingers dirty with.

Shepard meets the Illusive Man, but not face to face. She's scanned in by some kind of virtual projection device, and displayed as an image wherever The Illusive Man calls home. Shepard's been "upgraded", it turns out. The Illusive Man explains the dire situation humankind finds itself in. He wants Shepard to join Cerberus - an organisation she battled against in the first game - and investigate. Shepard, somewhat reluctantly, agrees. Cerberus, after all, saved her life.

Her first stop is a Terminus System human colony called Freedom's Progress, which has already experienced an attack. Now a level two Infiltrator, skill points are available. Do I improve Disruptor Ammo, which rips through shields and synthetics, or Tactical Cloak, which allows her to disappear for a short period of time? What about Operative, which improves her overall abilities? Choices... choices...

On the ground, it's deathly quiet, one of those "it's too quiet" situations. Then, a familiar face - the quarian Tali, who was a party member in the first game, turns up. She's looking for a fellow quarian, called Veetor, who mysteriously disappeared while on his pilgrimage. After some excited formalities and shock at Shepard's new allegiance, Tali decides to accept your offer to help find her quarry.

Mechs large and small attack; hugging cover, dishing out commands with the d-pad and using your own is effortless. It is a wonderful feeling to have Miranda "Pull" a bad guy into the air then yourself use the sniper rifle to shoot it out of the sky. Occasionally Shepard peeks out for some deadly sniper headshots and limb disabling - both satisfying combat improvements. As you hide behind cover, the game asks you to send your party members, with the left and right d-pad buttons, to each hug one side of a large door. A huge mech turns up, one you are told in no uncertain terms will pose a significant problem. Not really. The grenade launcher and Overload are enough to make it fall apart like so much junk.

Mass Effect 2 is bigger than Mass 1 in almost every way

Veetor is holed up inside a cabin, babbling some nonsense as he frantically presses control panel buttons and whizzes his eyeballs from screen to screen. He's not listening - perhaps he can't - to you. Now: the first chance to use the new dialogue interrupt system. With a press of a trigger, Shepard fires a shot that whizzes past Veetor's head. His attention is gained, but he's still out of his mind. "The swarms took everyone", he says with a demented panic. The kidnappers are revealed to be The Collectors, an alien, insect-like species from somewhere beyond the Omega 4 Relay. Somehow, they're involved with the Reapers. Somehow, they need to be stopped.

But first, what to do with Veetor? Tali wants to bring him home. Miranda, on the other hand, wants to detain him for questioning. It's your choice. Shepard, in this case, agrees with Miranda. Tali is upset, and leaves in a huff. If Shepard agrees with Tali, will she then join you on your adventure?

Back at base, you report in to The Illusive Man. What of your old party? Liara is working for the Shadow Broker, Kaiden's still with the Alliance, Tali you've just met, and Garrus, well... he disappeared after Shepard's "death". Shepard wants to gather together her old team - they will be perfect for the job, she reckons. And who can argue with her? They didn't do too badly in the first game. Whatever, it's time for Shepard to set out on her quest to save the galaxy, and in what better method of transportation than a new and improved Normandy?

For more on the new Normandy, check out our gargantuan interview with project director Casey Hudson, wherein he reveals loads of juicy titbits. We've had a fiddle ourselves, chatting with Joker, who's now walking (no doubt the result of some Cerberus tinkering), and conversing with the new ship artificial intelligence, EDI, which stands for Enhanced Defence Intelligence and is pronounced "Eedee". EDI admits to being the Illusive Man's spy, and to having some "restricted functions" even she's not aware of. She can, however, give enemy ships viruses, and disable their weapons, shields, gravity and air supply. We also spent some quality time gawking at Yoeman "call me Kelly" Chambers, your gorgeous green-eyed personal assistant and ship's counsellor (is she a potential love interest? BioWare won't say).

There's absolutely loads to do on the Normandy, too much to fit into the back end of a two-hour hands on session (I tried, but was booted off so someone else could have a go). But I did manage to poke about every level before giving up the controller. The Captain's Quarters is home to the fish tank, your Achievements and a flushable toilet. The Armoury is where you'll customise your weapons and those of everyone else's. The Private Terminal is where you'll check out status updates on your crew, read emails (Admiral Anderson wants to talk to you on the Citadel) and view advanced combat training videos. The Tech Lab, only available once you've convinced "The Professor" to tag along, is where you'll research new upgrades. And then, of course, there's the new elevator, which, somewhat sadly, is now a loading screen that displays a schematic of the ship, rather than an actual elevator with lounge music and news reports to enjoy.

Could Mass Effect 2 be 2010's game of the year?

BioWare's pushed the improvements it's made to the third-person shooting in the run up to release, but, having played Mass Effect 2 for an entire afternoon, it's clear that this only paints a tiny portion of the bigger picture. Compared with the first game, there's so much more to see and do and so much more to sink your teeth into, that it seems impossible to know where to start. We haven't even touched on the new Dragon Age: Origins-esque loyalty system (the snarky Miranda begins the game with "normal" loyalty, but I can see it degrading quickly). Nor have we mentioned the new ship upgrade system, which affects epic space battles in some exciting, unannounced way. And the Galaxy Map... well, who knows what magic wonderment that will bring?

What more is there to say? Mass Effect 2 is my most anticipated game of 2010. It should be yours, too.

Mass Effect 2 is due out on Xbox 360 and PC on January 29.