Valve faced a daunting task when it decided to build a sequel to Portal. After all, how does one improve on a title that most gamers consider to be just about flawless?

Originally released as part of the Orange Box, Portal was – and still is – an immaculately constructed FPS puzzler, ingenious in its design and underpinned by a darkly humorous yarn. The only complaint you could level at it concerned its brevity, but even that didn't seem to bother many people. Short, smart and perfectly formed, Portal was the complete package. So what need did it have for a sequel?

And yet, in the next couple of weeks, Portal 2 ships and players will likely return to the Aperture Science Computer-Aided Enrichment Center – the human behaviour research facility from the first game – in their droves. Or rather, what's left of it. As the game opens, it's revealed that Aperture is a mess. The smooth, sanitised surfaces have been shattered. Rooms are filled with smashed debris and jungle overgrowth. The place looks like it's been hit by an earthquake. According to Portal 2's writer, Chet Faliszek, Aperture's facelift is Valve's way of establishing the game's story from the off.

"The first Portal kind of sneaked up on people," he smiles, "and we'd love to have done that for the sequel because we're really lazy and that would have been easy to do. But hey, the jig is up! Portal has a story. So instead, we want the player to know right off that something's wrong so we pound on their expectations. And we pound them with Merchant!"

Merchant, in this case, is Stephen Merchant, who provides the voice of the first character the player meets – a hovering droid named Wheatly. In the opening stages of the game, Wheatly reveals to the player that Aperture's been wrecked by "some sort of apocalyptic event" and steers them back into the training facility and towards a Portal Gun.

His dialogue is a stream of nervous pronouncements and laugh-out-loud gags, which apparently boiled out of some writing jam sessions involving Merchant, Faliszek and the rest of the Portal 2 crew. It's a nice way of establishing the game's black comedy; Portal was a story of how events can take a horrible turn for the worse if everything is left exclusively to logic – as personified in that game by GlaDOS, the malevolent female AI who tried to kill the player – and Portal 2 picks up that theme and runs with it.

Portal 2

"I just love the idea of a place where science and logic have been placed so far ahead of everything else," says Faliszek, "as if that's the only way human beings can advance. It's 'science first' and everything else is 'meh'!"

Once Wheatly plops the player back into Aperture's test course, the puzzle solving begins. The facility might be a wreck but its doors, pressure pads, elevators, forcefields and numerous other test apparatus still work. Naturally, the Portal Gun makes a return and so too do its mind-bending abilities. Players can use it to punch two portals that can connect most of the surfaces in their surroundings. Cubes are, thankfully, still in abundance when needed.

Over the first few levels of the game's first Chapter, the player proceeds through a brief tutorial disguised as some fairly easy tests. Gradually the difficulty level starts to ramp up and the solution to each new test room becomes more and more challenging. As in the original game, the puzzles in Portal 2 are pitched perfectly; beyond the tutorials, which are deliberately easy, none of the test rooms I encountered were too basic, yet none were eye-wateringly hard.

"We were aiming for more complexity," says Faliszek. "We didn't want to make the game too difficult, but we wanted Portal 2 to be more complex than Portal 1 and the only way to do that is through testing. And then more testing.

"We couldn't do it otherwise. We watch hours and hours of play-testing. We start with a basic idea for a puzzle, have some players run through it, and then depending on how fast theu solve it – or don't – we know what we need to tweak Maybe moving a wall here. Maybe making something a bit more obvious there …"

Once the first trials are over, the player is reunited with Wheatly, who guides them into the facility's breaker room – a massive rotund area filled with switches. Once Wheatly activates the room's elevator, the pair of the are pushed upwards to the surface and into a room filled with metal debris – and it's here the game's most famous character makes her return.

As Wheatly mutters away about trying to hack into Aperture's databases to allow himself and the player to escape, the wreckage behind him begins to reassemble itself. Within moments, GlaDOS's cracked sing-song voice rings out. She remembers the player from the first game – "when you MURDERED me" – and has decided to reactivate Aperture's more deadly test courses. After disposing of Wheatly, she dumps the player back down into the facility …

… and it's here, unfortunately, that the demo ends, leaving me with a whole host of questions for Faliszek about what more we can expect from the game.

Portal 2

"The problem with showing Portal 2's content beyond the introduction level is that it would give away a lot of the story and a lot of the twists," he says. "All we can do is show you a very compressed training course. There's a bunch of new stuff, but we can't show it all without giving away spoilers."

According to Faliszek, though, Portal 2 boasts a huge amount of content and mechanics to aid in the single-player puzzles. Aerial Faith Plates are springboards that send the player hurtling through the air. Propulsion Gel, which allows the player to slide quickly across floors and build up massive speeds. Repulsion Gel, which allows the player to create areas they can bounce on to cover huge distances. Excursion funnels, which operate like anti-gravity travel tunnels that can be directed around the map with the Portal Gun.

Faliszek says that each new element will be introduced with a couple of easy levels, so players can familiarise themselves with the mechanics, and then, naturally, the difficulty will rise.

Finally, Portal 2 also boasts a two-player co-op mode, which in an age where every shooter seems to have to contain one, doesn't sound tacked on or forced. The co-op mode features two new characters – two Aperture Facility bots – and a standalone story that is separate from the single-player as well as its own set of maps. In it, two players will have to work through a series of tests that can only be solved through successful teamwork.

"The co-op kind of came from Portal 1," says Faliszek. "You'd be sitting on the couch and your friends would be sitting around you and they'd be helping you solve the game. So we wanted to create a situation where at least one of them had a controller too."

Indeed, it looks like a lot of Portal 2's content – from its new mechanics, its new multiplayer mode to the long list of well-balanced brand new puzzles – seems to have been made possible through Valve's close studying of the behaviour of its game's audience. By all accounts, the developer sounds a little bit like Aperture itself – minus, of course, any lethal tendencies.

"We never inflict actual pain on our audience or testers for real," laughs Faliszek, "as much as we would have liked to."

• Portal 2 is out on 21 April on PS3, PC/Mac and Xbox 360