The following review is spoiler-free.

Fear not: this review has been the subject of endless scrutinisation to ensure that you, our cherished reader, do not have one of the best experiences of 2011 - nay, of recent years - ruined for you in any way, shape or form. To that end, there are no plot spoilers, no dialogue quotes, no references to cake or lies, no lyrics from Still Alive used as cheesy puns and no explanations of what happened at the end of the first game (for those that weren't intellectually on the ball enough to finish it). All of which has made this a right bugger to write.

It begins in what appears to be a motel room; a quaint little cubicle furnished with a bed, desk and the odd decorative object. You've been asleep for some time, hauled out of a mammoth slumber by an energetic banging on the door and a familiar voice. It's that Bristolian chap from the Barclays adverts, Ricky Gervais' pal - Stephen Merchant. Only here he's a spherical robot with a blinking blue eye - something known as a Personality Sphere. Despite the simplicity of his design - a lack of any distinguishing features at all, really - Wheatley possesses more personality than the cast of most other games put together.

I think he might be my favourite video game character of all time.

Valve has an uncanny knack for bringing life and colour to even the most mundane of objects. Remember the Weighted Companion Cube? An inanimate block that was little more than six uniform faces decorated with love hearts? Of course you do, because you became inexplicably attached to it, then ashamed of the cruel, cruel thing GLaDOS forced you to do to it. Such is the power of Erik Wolpaw's script-writing talents, which are of an even higher calibre the second time around. Portal 2 raises the bar for dialogue delivery, presenting a continuous stream of hilarious chatter, witty insults and comical references to past antics. All of which I can't talk about.

It's a distressing situation to be in as a games journalist, because all the best bits in the game - the bits that justify the hefty score at the end of this review - are all closely intertwined with the narrative. You'll just have to take my word for it: the characters, dialogue, and twists and turns of the plot are Portal 2's shining accomplishment.

I assume you'd like something in the way of contextualisation, however, so here's the horrendously vague gist of it all. That motel room I mentioned at the start of my review? You break out of it and soon find yourself back in Aperture Science labs. It isn't long before you're the subject of a familiar testing regime once again, a plaything for GLaDOS, the potty AI from the first game. And that's all I'll say on the matter.

So, let's talk puzzles - the test chambers designed to put your ability at thinking with wormholes to the test. Armed with your trusty portal gun - blue portal on the right trigger, orange on the left - you'll need to get from a starting point to a designated exit, with the space in between stitched together with all manner of button-pressing, bottomless-pit-crossing, laser-redirecting and killer-turret-avoiding. In order to succeed, you'll need to set up your portals in the right places at precisely the right time. This requires an out-of-the-box approach to thinking and a relaxed attitude to abstract concepts. Even after four years, I still find it totally bewildering to walk through one portal and emerge from the other in a completely different location.

The first Portal was undeniably brilliant, but most of its puzzles were variants of the same handful of concepts. Portal 2 is a far more diverse experience in this respect, introducing a host of new mechanics and subsequent applications of your portals.

Surface modifiers are the most notable example of this, which give new properties to a wall or floor. Blue Repulsion Gel, for example, lets you bounce to lofty new heights, while the orange Propulsion Gel boosts your speed, letting you zip through one portal and emerge from the other with enough pace to cross otherwise impassable chasms. Conversion Gel is slightly different, allowing you to paint over a surface so that it can support wormholes. Latter puzzles bring together all three gels, which turns a test chamber into a scene from an infant school art lesson.

Light Bridges allow the creation of tangible walkways across test chambers. Pop a portal on a wall at the destination of a light bridge (they flow from a source, kind of like a river), and the walkway will continue to beam out of your second portal, wherever that may be. Excursion Fans work in a similar fashion, although they lack the same tangibility. They're best imagined as air vents, magically carrying anything caught in the stream until something blocks its path.

Portal 2 doesn't just welcome diversity in terms of its inner mechanics. Where the original game maintained a clean and clinical feel from start to finish, Portal 2 presents imperfect and structurally deformed test chambers, areas overrun with vegetation, and others tainted by human hands. Subsequently, the adventure feels far more grandiose, with the weight of the narrative supported by a range of impressive environments and settings. Obviously I can't talk about where things head towards the end of the game, but OH MY GOD.

Co-op, then. As the Laurel and Hardy-inspired droids ATLAS and P-body, it's your job to complete cooperative test chambers under the watchful eye of fellow AI (and creator), GLaDOS. She explains that the idea behind these tests is to observe how well a pair can work together to tackle a shared problem, and she dishes out Science Collaboration points based on your successes.

A running joke throughout the game finds you rewarded with points for successful actions, and penalised for botched jobs - though it soon becomes apparent that the entire system is yet another flourish of the wonderful script. GLaDOS revels in the competition between the two players, dishing out insults to anybody she perceives to be slacking, and praising the other in order to incite jealousy. While there's no narrative to tie everything together as there is in the single player campaign, GLaDOS' commentary on your activities is as amusing as ever.

With each puzzle demanding increasingly complex solutions, Valve has had to implement a means of allowing players to convey their strategies. A Ping tool lets players paint a marker on a surface or object, suggesting that their partner should whack a portal on it. Anybody attempting to play the game without a headset will find this indispensable, but those playing in the same room (the game supports split screen play) will find the tool equally as helpful. It might be simple, but sheer genius in terms of how it permits the simple communication of ideas.

The co-op half of the game is considerably harder than the single player, as far as I'm concerned, which makes sense given the whole "two heads are better than one" shtick. Being stuck on a puzzle was never something that bothered me, however. The process of discussing potential solutions with your partner and trying out different strategies turns out to be the most enjoyable thing about co-op play.

Valve knows how gratifying it is to nail a tough puzzle, and prompts players to celebrate their intellectual prowess in game via a gesture system. Pressing up on the D-Pad will bring up your gesture wheel, where you can choose to wave at each other, high-five, dance and even play Rock, Paper, Scissors. It might seem a trivial addition, but - on top of allowing players to interact with one another and celebrate - it really helps bring the two droids to life. I suspect many people will find ATLAS and P-body to be the biggest stars of Portal 2.

Unlike more traditional co-op, you're not just deciding who shoots which alien with what gun; you're carefully planning and executing a very specific strategy, an impossible task unless both players are doing precisely the right thing. It's rare that a game brings together two players in such a way, which makes Portal 2's co-op one of the best multiplayer experiences I've ever played.

I think you knew all along this was going to be great. Fantastic, even. All the same, I don't think you'll be prepared for it being this good. I may have buttoned my lip in terms of concrete spoilers, but a mere ten minutes here will yield more memorable moments than you're likely to squeeze out of the entirety of most other games this year. In all aspects of its design, Portal 2 is genius.