If the idea of Portal 2 spoilers fills you with DREAD and TERROR, please hollow out the nearest Weighted Companion Cube and stick your head in it. Or look away. I had it spoiled for me by Valve of all people, who sat me down and forced me to play the first 45 minutes of their dark, comedic FPS puzzle game. For science. Those monsters.

It begins in a hotel room. Over the tannoy, an unfamiliar voice – male, flat, not brimming with psychosis – explains that I'm part of an experiment, and they need to check my mental wellbeing. I'm told to appreciate a painting on the wall, (“this is art”) before being ordered to bed.

I awake to a broken room, the relaxing, neutral colours smeared with grime. The recorded voice tapers off: “you have been in suspension for 999999...” then another, more characterful voice chimes in. Wheatley, one of the leftover personality spheres of the wrecked GLaDOS, is trying to get my attention. His nervous English voice, that of Ricky Gervais' comedy partner Stephen Merchant, is another indicator that while Valve might not have known what they had with the original Portal, this time around they're a lot more confident. Wheatley isn't the spare, precise, GLaDOS: he's chatty and lonely and needs my help. Chell is all he's got left.

My room is moveable. It's one of 10,000 in a vast rack. Wheatley moves it, chatting all the while as it swings and crumbles. The more holes that appear, the more I see of Aperture Science beyond. It's big. Black Mesa big. But the walls have fallen from neglect. The outside world is vaguely visible through these gaps, a hazy glow in the distance.

After Wheatley applies a brief “manual override” to an inconvenient wall (he uses my room to bludgeon it down), I scramble into the facility.

The portal gun awaits, as do some of the old test chambers, but hundreds of years of neglect have changed everything. There's life where there were once sterile white walls. Birds caw, the light feels natural, everything's tangled in weeds. In a repeat of the first game's tutorial, I get a portal gun with just one function: placing blue portals. But the game is a lot less cagey about introducing me to the complexities of the tests: three puzzles in and I'm led to one where I need to pop a portal under a cube to get it over to a super colliding super button that leads me to two more switches, one that drops a cube on a slanted panel so it slides into a vat of sludge, another that flips up a section of the floor. I pop a portal on the slanted panel and the cube flies through the fixed portal on the wall above, into the sludge. Next try, I flip the floor section up and it stops the cube flying into the sludge.

So it's a lot denser than Portal. Things get more complex a lot sooner, and the world is almost overwhelmingly detailed. There are warning videos everywhere, one warns of a potential 'Animal King Takeover'. A dangerous puzzle is accompanied by light jazz to make it more relaxing. In one room there's a story mural of the history of Aperture Science, showing the moment GLaDOS took over and Chell's later defeat of her. There's a giant painting of Chell, as if whoever's been left behind worships her as a god.

And three characters are introduced: the flat voiced, male anti-GLaDOS, Wheatley, and eventually GLaDOS herself. She wakes (“We've both said a lot of things you're going to regret.”), dumps me into the incinerator room and talks of living her death over and over and over again. She was already mad, now she's focused. This, I guess, is where the new puzzle elements will start coming into play, such as the special gels. She has all these new tools to mess with, but I'm not allowed to see those yet. The rest of the game will remain a mystery until release.