Time has passed since the first Portal game - Aperture Labs, destroyed at the end of the first game, has long been reclaimed by nature. The labs are smashed, trees and vines invade the once clinical space and GlaDOS is dead after you killed the psychotic CPU and escaped the rat maze of Aperture Science. In Portal 2 you return to play as Chell, the character from the first game.

But what can you expect to see when we return to Aperture?

Well, first up there's a new ally character - Wheatley - "he" is a spherical CPU like GlaDOS but with an English accent. He appears to be quite shy - he won't let you look at him when he's perfoming tasks, for example. Carry him around, and he talks to you, place him in access points and he can open doors and reveal secret areas. Wheatley epitomises what Valve means when it talks of 'recapturing' the Portal spirt of the first game: namely the humour and the attachment to the game world that we all loved so much.

It's all summed up quite nicely in the scene when glaDOS reanimates herself. You've just carried Wheatley across a series of simple puzzles, to arrive in glaDOS's old lair. She begins the boot up sequence just as you arrive and Wheatley tries to hack her systems to prevent it. This being Portal he starts trying to guess passwords with AAAAA, then tries AAAAB... and then it's too late: GlaDos is still alive. Hilarious.

There are plenty of new additions to the number of devices and powers in the game, each suitably titled as if GlaDOS herself had chosen their name.

Aerial Faith Plate is basically a super-power jump pad, that can propel you and objects; the Thermal Discouragement Beam is some kind of super hot laser that orignates from panels in walls, floors etc. It burns rather than shoots (and potentially evaporates liquid) objects. It can also be directed with the prism-like reflector blocks. And, naturally can also be directed through portals. The Phenumatic Diversity Vent is essentially a huge vacuum tunnel that'll suck up and dispose of anything placed in it. Portals can be used to suck things - blocks, gun emplacements and so on - into it and clear the way. It also has the suction power to remove panels from walls to reveal secret areas.

The Repulsion and Propulsion Gels are two liquids that dramtically change the properties of the surfaces that they cover. Repulsion lets you jump higher, and will bounce off surfaces when you fall from heights while Propulsion works like an oil slick - speeding up your movement. Both Gels can be used to 'paint' areas using the Portal Gun and are chiefly used in conjunction with other elelments to solve puzzles.

Finally, Excursion Tunnels can be created by the player and are basically energy tunnels that can transport the player and objects - which and can be entered anywhere along the tunnel.

Overall, Portal 2 has many more complex levels - the aim, says Valve, is to teach the player how to use each of the new elements and then to bring them all together in multi-layered single levels.

These levels - due to the nature of the situation (GlaDOS is rebuilding the labs) will also change dynamically - sometimes cinematically, sometimes as you're playing in game. Valve told us that the game as a whole is somewhat more open, but Portal 2 retains a similar structure to the first game - it is at least twice the size of the original.

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