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Ok, so you could put our social lives into question at this point (we managed to sneak in a few beers, promise), but the explanation is a just one; Bioshock is one of the most immersive, extraordinary shooters we've played since, well, Half-Life 2 - and, unless you've got the motion sickness problems of a Turok Q&A tester, you just have to play it.

It kicks off in 1960 with your plane crashing into the sea. After a cinematic bathysphere ride you arrive in Rapture, an underwater city where the world's greatest minds have escaped the constraints of ordinary man. But Rapture's doctors, inventors and entertainers all suffer from the odd mental flaw; cracks which became gaping holes once the city's extensive genetic mutation products, called 'Adam', went into production.

Rapture's inhabitants were consumed by mutation and their obsession with Adam soon turned into civil war; brother turned on brother, babies were strangled in cribs.

Now all that's left is a crazed and demented populace of 'splicers', changing and enhancing their twisted bodies with all the Adam they can get, and a city moodier than Bianca from Eastenders at that time of the month.

Rapture is the star of Bioshock and it's one of the most extraordinary and atmospheric game worlds we've ever explored. After what feels like an eternity of space marines, knock-off Middle-earths and WWII beaches, 2K's underwater city is a refreshing and disturbing change of scenery.

Every corner of the environment has been carefully considered to project a living world with an unsettling aura that only the spiritual sequel to System Shock 2 could deliver. Water - amazing-looking water - pours from cracks in the ceiling. Debris litters empty apartments and the shadows of rambling, mental inhabitants haunt the empty hallways.

The demented, well-acted (uncomfortably so) people of Rapture might not be as animated as Half-Life 2's cast, but they'll leave a bigger impression on your mind. Splicers scramble with fish hooks pointed at your chest, screaming incessant cries about their lost children. It's all very unnerving. Rapture's different districts back this eerie feel up too ranging from gritty, damp factories to underwater forests. This really is a beautiful looking game.

A compelling narrative and a well-realised and believable game world are already enough to create a classic, but over a length of time a good story isn't enough to keep you entertained. Thankfully 2K hasn't overlooked the traditional shooter elements of Bioshock, it is an FPS after all.

On its own Bioshock's gunplay wouldn't change the world of FPS; very quickly you have a revolver, shotgun and machine gun in your inventory but thanks to the limited supply of ammunition you have to continually hunt for, weapons feel a lot more empowering than in most other shooters.

You'll never have enough ammo to blast away with abandon. If you don't want to be left with the wrench (which, funnily enough, quickly becomes one of the most useful weapons in the game) you'll have to search every desk and spend every last dollar in the liberally scattered vending machines for bullets.

In the same way as Resident Evil, this ups the tension. To make it even worse, there's 16 different types of ammo in the game, three for each weapon. This lends even more weight to your decisions and where you're going to spend or conserve ammunition. We favoured conserving grenades and armour-piercing rounds for larger enemies, while the napalm, liquid nitrogen and electric gel ammo of the chemical thrower has vastly different effects depending on who you use it on.

Complimenting the usual array of guns are genetic powers called 'plasmids', similar to System Shock's psychic powers. Plasmids are controlled with the left trigger and bumper with weapons on the right. You soon find the electricity bolt, fireball and telekinesis powers, but the scope of these bolt-on upgrades is gigantic.

Plasmids are great to weaken and trick enemies, so a bit of back and fourth between guns and powers is essential. The interesting part is that you gradually create your own combat style. The Enrage plasmid tricks splicers into killing each other. You can use scenery to your advantage too; setting oil slicks aflame does the job a treat.

The deepest and most definitive part of the formula is the simple but involving RPG element. There are so many plasmid combinations and choices that a second play-through is guaranteed. Character and item building options are accessed via vending machines. Rarely-found 'Power to the People' stations give a one-time weapon upgrade ranging from damage increase to a larger clip size. Inventory stations - a nice idea - can similarly be used to build extra items such as ammo using bits and bobs scavenged from your journey.

Hacking also plays a major role and even these skills can even be upgraded through plasmids. Rapture's security system is immense and involves an entire network of cameras, turrets and flying bots that can be yours if successfully hacked. Oddly this is done through a complete rip off of Pipe Mania in a quick mini-game. You can even hack vending machines for cheaper goodies, which saves your wallet from taking a financial beating.

As for your plasmids and modifying tonics (which affect things like your max health and engineering attributes), you'll get those from special Gather Gardens - and the only way to craft them is by using Adam as credits. There's only one way to get it, and that's through the Little Sisters. And where there's a Little Sister, there's a Big Daddy...

Left alone these gigantic, diving-suited guerrillas are the most harmless inhabitants of Rapture. Cross them though, and you'll come face to face with one of the angriest characters we've ever encountered. You won't survive long enough to shout about it either.

Big Daddies guard the twisted, syringe-wielding little girls that harvest the only source of Adam in the game (there are two or three of them in each district, marked on the pause screen). Their relationship is both unsettling and touching at the same time. Little Sisters draw the Adam out of corpses, Big Daddies provide the backup.

These twisted little girls will skip, hold hands and beckon their metal protectors through Rapture's blood-splattered plazas, and the Daddy will give everything he's got to hug, protect and defend his tiny companion from anyone foolish enough to get too close. But if you want to bolster you're plasmids, you're going to have to piss him off.

The rules of gaming state that a hulking, heavily armed foe will be slower than a milk float with a flat. Safe in this fact, you'll likely unload a revolver in his face from 30ft down the hallway, and then shit your pants when he rushes towards you in a single, blurry dash before you realise you're still alive.

When you do die though Bioshock's death system brings you instantly back to life in a resuscitation chambers, meaning you can go straight back and continue the fight - with your enemy's health at whatever point you left it. Admittedly, this takes away some of the difficulty and tension of taking on the giants as there seems to be no real penalty for dying.

There's a dark story twisting its way through the world of Rapture and the game throws emotional choices onto your lap. Once you've defeated a Daddy and the Little Sister's crying next to the smoking shell, you've got a choice on your hands - save the little girl, taking one good thing from this dirty mess back to the surface, or harvest her, taking maximum Adam to tone your powers, but killing the child in the process. The consequence of either harvesting or saving a Sister only really affects what ending you see.

Like Valve's Half-Life 2, Bioshock doesn't rewrite the rules of first-person shooting, but it exceeds in both narrative, world and gameplay - which, despite how many times you may have fired a machine gun or built up your stats in the past, makes for more single, standout moments than we've had in a long, long time.

Moments like the countless twists and set pieces in Bioshock's fantastically well-written plot.

Or the huge variety of combinations you can play with, or the many interactions with the Big Daddies you'll have, like accidentally catching them in your crossfire and having to leg it across a whole level to escape their screen-shaking charge.

The world of Rapture is almost enough to make Bioshock an exceptional experience on its own. But the fantastic role-playing elements, standout scenarios and truly mental character encounters make it extra special. An experience of this calibre only comes around every few years and should be played by as many people as possible.