Few sequels face as daunting a legacy to live up to as Portal 2. The original stands as one of the cleverest and most revered games ever made and so the pressure is on for the return visit to Aperture Laboratories.
As before, you play a human lab rat subjected to the experimental challenges dreamt up by the evil computer GLaDOS. The goal is to exit ever more puzzling rooms with the aid of a Portal Gun that lets you shoot up to two virtual doorways onto walls coated with a special substance. Once you've got two portals in place you can walk through one and come out of the other.
Initially the challenges thrown at you are straightforward: zap one portal onto the wall next to you, put the other onto the wall next to the inaccessible exit and then simply walk through the portal. But after a few gentle introductory tests to familiarise you with the concept, Portal 2 starts presenting you with brain-aching challenge after brain-aching challenge.
You might need to open an exit by placing a block on a pressure pad, but the block is located on an inaccessible ledge high above you with no obvious route up. To reach it might involve using portals to reach a high walkway before firing a portal into the floor below and the other into the wall opposite the ledge with the block, so that when you jump down into the portal on the floor you are sent flying out of the other portal and onto the ledge with the block.
And soon the complexity is cranked up with the addition of other elements to the puzzles such as tractor beams, gels that alter the properties of surfaces and bridges made out of light.
Like many puzzle games the description often sounds more intimidating than the reality, but Portal 2's environmental conundrums are truly challenging: requiring lateral thinking and dissolving notions of up, down, left, right, forward and back. But while the challenges can be bewildering, the sense of achievement you get from solving a challenge that had you stumped is immense with confusion melting away into a thrilling Eureka! moment.
Portal 2's fiendish puzzles would make a great game on their own, but the game doesn't compromise on setting either, moulding a captivating story out of its puzzling action. The script and voice acting is nothing short of fantastic: the acidic wit of superbitch GLaDOS balanced by the bumbling Bristolian tones of Stephen Merchant, who plays an equally amusing rival AI.
Beyond the single-player mode there's a set of co-operative two-player challenges that can be played in split screen or online. The two-player challenges are even tougher than the single player so the need to discuss with each other how to tackle each puzzle is paramount. While this means it works best when players are in the same room, Valve have done a sterling job of providing players with tools to indicate their ideas to other players so that the online mode works almost as well.
The only possible criticism you could level at the game is that once a room is solved, there's little reason to go back. But when a game delivers as many moments of joy and satisfaction as Portal 2, you'll need to be a rather sour soul to make such a charge.