We’d been slaving away at this test chamber for what feels like an eternity. My best friend carefully places two portals on a ledge, redirecting the light bridge I was standing on to point towards the exit. Of course, I was not redirected myself, and so, as my closest friend laughed on the sidelines, I plummeted 50 feet below into a pool of acidic goo. Then I came back, we hugged, and set aside our humorous differences to work at actually finishing the level.

Such is the co-op of Portal 2.

You play as one of the two quirky robots ATLAS or P-body, and the person you’re playing with becomes the other. That person can be next to you with the game played in split-screen or over the internet via Xbox LIVE, PlayStation Network or Steam (this even cross-platform between PS3 and PC players, for the first time ever), so you’re probably going to be able to find a friend you can co-operate with.

The gameplay is identical to Portal 2; you’ve still got a portal gun, and when you fire it, it still creates two linked portals through which you can place yourself, an object or a gameplay device (such as an ‘excursion funnel’, a tube made of liquid asbestos that carries you through the air, or the aforementioned hard light bridges). The difference, of course, is that there are two of you, so that’s four portals that you have to manage. And these test chambers are hard, too – this isn’t just more single-player levels with another guy there to watch how awesome you are. Both of you are needed, and your portals need total co-ordination.

This would be very prone to levels descending into shouting matches as players desperately scream “Over there!” to each other, had Valve not included the “ping tool” that allows you to precisely indicate to your partner an object of interest (for instance, your favourite animal in the tutorial). It has a number of settings, such as “put a portal here”, “look here” or “go here”, but you really need a headset to play this properly. With verbal communication, all that’s needed is for you to use the ping tool to indicate a location while you discuss it. However, the idea itself is absolutely essential to play the game, and without it things would doubtlessly get a lot more difficult for all the wrong reasons.

Not only are the Portal gameplay features you know and love all back here, but so too are GLaDOS’s ever-present sarcastic quips at your expense. This time, she’s growing intensely disgusted at how human the two robots are becoming, and schemes to drive them apart as they make their way through the tests and towards GLaDOS’s more sinister designs. At first, she awards and deducts meaningless “Science Collaboration Points”, before moving on to more “cunning” (read: childish) agendas – but the two robots bond over the course of the game, and you get to experience that through several gestures such as waving, hugging and playing rock-paper-scissors (the latter of which I always somehow seemed to lose).

ATLAS and P-body are brilliantly animated, exaggerated to the point of hilarity, and the addition of a “splat” animation for when they smash into something really hard has to be one of the funniest things of the entire game (especially when you definitely accidentally send your partner into a loop of repeatedly bashing their head on the ceiling). There’s no fun in working towards a goal with someone else if you don’t feel a connection to your partner – and Valve certainly does a good job of making sure you’re both friends (even as GLaDOS and the tests themselves might prove otherwise).

I played the co-op mode of Portal 2 with a friend in the same room as me, him on a PS3, me on my PC. The co-op mode clocked in at about 8 hours long (the same as the single-player and twice as long as the original Portal), and both versions ran smoothly with only a minor difference in loading times, owed mostly to my aging computer versus the PS3. There was absolutely no lag, even across platforms, to the extent that we could both have the sound on and GLaDOS’ voice matched up perfectly. That makes Portal 2’s co-op a technical achievement, not forgetting the massive sense of achievement you get when you finally figure out its insane puzzles.