In many ways Mass Effect is an unlikely success story. Starting out as a slightly awkward role-playing game with big ideas wrapped around a surprisingly deep - and, let's be honest, rather nerdy - sci-fi core, BioWare's second outing saw the series somehow evolve into a bona fide blockbuster. Two years later and the end is near for Commander Shepard and crew in their battle against the Reapers, the race of sentient machines bent on wiping out all organic life in the universe, and BioWare is determined to go out with a bang.

Naturally, that means more shooting. If the original game was a very clearly-defined RPG that happened to feature tactical gun-based combat, and its sequel tilted more in the direction of the latter, Mass Effect 3 can sometimes feel like a third-person shooter with dialogue interludes. This assumes you whizz through the story missions and ignore much of that wandering-around stuff, but it's telling that there are options to streamline the role-playing mechanics significantly, and even for the game to make all choices for you. That might be a useful option for the indecisive among us, but it speaks volumes of BioWare's intent for its combat to match up to genre standards.

Yet judged as a shooter, Mass Effect 3 is enjoyable but unremarkable fare. There's a larger selection of weapons than its predecessor, all of which can be upgraded and modified, and some of them are enormously good fun. Most new guns are salvaged during missions, and you'll soon pick out a few favourites, like the burst-fire SMG, or a wonderfully messy shotgun that takes ages to reload but can take even armoured enemies down in a single blast. Though you'll sometimes find them trying to flank or flushing you out with grenades, opponents tend to overwhelm you in numbers and power rather than any strategic effectiveness.

It's up to you to shake things up with squad commands and the use of biotic powers, because while BioWare knows how to stage action, it's less confident in balancing and varying it. A great many objectives revolve around protecting a crucial asset or holding a position for a certain time, and while turret sections, boss battles and the odd shooting-from-a-moving-vehicle interlude are welcome twists, these are all still pretty familiar ideas. Meanwhile, snapping into and out of cover can be something of an imprecise science, and when your shields are down and you're low on medi-gel, that's the kind of minor irritation that can prove fatal.

But, of course, the combat is just one element of the Mass Effect experience. This represents the culmination of a tale spread across three games and dozens of hours, a story finally coming to its dramatic climax. And what a story it is. It's not necessarily that it's well-written - though it often is - but more that it's well-orchestrated. This is a script moulded by its players, a journey guided by decisions you might have made two games ago. Heck, even if you're coming to the series afresh, you get to choose a backstory for your male or female space hero, before making a couple of key decisions veterans will get to bypass, chiefly whether you sacrificed whiny boy band member Kaidan or beautiful racist Ashley.

Somewhere at BioWare there's an enormous flow chart with every decision branching off in another direction, circles and lines headed every which way, keeping track of how it all slots together. It's a remarkable achievement and it helps make every player's journey feel personal. This idea even feeds into cutscenes: it's quite something to witness your Shepard wearing the armour you've equipped coloured in the tints you picked out carrying the gun you just modded and talking to the squadmates you chose at the start of the mission. Sure, for the most part you'll be visiting the same planets and taking in the same set-pieces, but who you take that journey with and the roles they play alongside you changes significantly. It's similar but different, just enough to encourage discussion of commonalities and differences with other players.

BioWare also proves itself a master of context, making every step you take feel important. Shepard might be a bit of a blank slate (that's certainly true of male hero, perhaps less so of his spunky female counterpart) but he or she is a hero who people look up to. Make a difficult decision, and you're constantly reassured that you've made the right choice. Elsewhere, characters will frequently let you know how honoured they are to serve under you, that you're the only one brave and capable enough to repel the Reaper threat. It's hugely empowering.

Mass Effect's other ace in the hole is its willingness to up the stakes. It's never a choice between brutally bludgeoning someone and presenting them with a bouquet of red roses; instead, these are the kind of dilemmas that have you putting the controller down, scratching your head and mulling over for minutes at a time. After all, more often than not, they're a matter of life or death. Like a TV exec brought in to shake up an under-performing soap, no one is safe from BioWare's scythe.

It helps make even the smaller decisions seem significant. Here, you're frequently forced into uneasy alliances, trying to keep previously warring factions sweet to unite against a common foe. If the first game set you as a fairly standard world-saving hero, and the second as intergalactic relationship counsellor, flitting between planets on missions to cajole others into committing to your cause, here you're a peacekeeper, trying to avoid a war breaking out aboard the Normandy just as the battle to save the universe rages outside the airlocks.

As such, the decisions you make here are all a means to a noble end. You can play the virtuous hero, constantly appealing to everyone's better nature; or you can opt for the results-oriented approach, issuing blunt ultimatums, shouting, and getting your hands dirty for the sake of saving life as Shepard knows it. Whether you're Paragon or Renegade, the points you earn count towards a total reputation figure, ensuring you don't try to game the system by picking all 'good' or all 'bad' options to unlock the best abilities.

Meanwhile, BioWare sensibly grounds the action to convey a greater sense of threat. The first time we see the Reapers, they're descending on Earth, and it's not long before they're branching out. Before long, your galactic map will be dominated by these metallic hermit crabs, and their presence has an effect on your approach to mining. What was a bizarrely compelling bit of mindless busywork in the last game is now a careful side pursuit, as you send out signals in the hope of receiving one in return. Should you hit paydirt on a planet, you can start searching and send out a probe, only this time you're not searching for minerals but for war assets which could prove crucial in the final reckoning. Yet every time you ping a signal, the Reapers will start to trace it; do it too often and they'll enter orbit accompanied by a terrifying, otherworldly sound. At this point you'll need to escape to the safety of another cluster, though if you're caught, you can merely resume from where you left off, a rather feeble punishment that lessens the sense of danger. Before long, these searches feel too much like hard work.

Attempting to manage your mission codex can be something of a chore, too. Walk past NPCs and their conversations will register key data in your codex pertaining to sidequests. Often, they can be as simple as retrieving an artifact from another planet through mining, or obtaining an item elsewhere. But they remain highlighted in your mission summary until you complete them, and for many, the pile-up will prove alarming. Worse still, there's an unseen time limit to many of these and it's easy to miss them because you opted to tackle something else first.

Then again, BioWare does manage the difficult task of making even the most minor aside feel like a crucial part of the bigger picture. Before you could be forgiven for wondering why you were titting about when the galaxy needed saving, but here everything you do contributes to the war effort. Sure, researching a medi-gel upgrade might seem small fry next to the possibility of an entire race being wiped out, but there's a tangible reward for every minor action that could potentially change the tide of battle against the Reapers.

Elsewhere, it isn't afraid to dream a little bigger. BioWare has broadened its canvas when it comes to set-pieces, and though its ambitions occasionally clash with the limitations of the game engine, the results are often spectacular. An early highlight has you fighting on the surface of a moon while its mother planet burns in the background, the first staggering sight of many. Replacing Jack Wall was a controversial move, but Clint Mansell's plaintive piano themes prove the perfect backing for the game's most moving moments.

The multiplayer game is also far from the perfunctory check-box addition you might expect. Again, context is key: these co-op missions tie into the narrative with levelled-up characters contributing to your galactic readiness. It might be little more than a Horde mode with additional mission objectives besides 'kill everything that moves', but it's surprisingly well-constructed.

There is, in short, a hell of a lot of game here, and at a time when free social and smartphone games have analysts wondering aloud whether full-priced retail games can cut the mustard, here is a package that represents exceptional value. There's little here to convert non-believers, but then this game is not for them. This is one for the fans, and few who buy it will be left unsatisfied by how the story - their story - ends.

Version Tested: Xbox 360