Surely there are few groups with a greater willingness to suspend their disbelief than the people who play video games. Whether we're pretending to be commandos in follow-the-leader first-person firework displays like Call of Duty or a petal floating on the wind in a game like Flower, we're always on the front lines of make-believe.

At first glance, the BioShock series seems to demand a greater will than most. The original game took place in a city at the bottom of the ocean where you could inject yourself with a blue liquid that let you fire bees out of your hands. The latest one, BioShock Infinite, is about a flying metropolis called Columbia where people travel around by hooking onto a network of overhead rollercoasters.

Your job is to rescue an innocent girl who seems to be able to rip through time and space and is being pursued by a giant mechanical songbird. Oh yeah, and it's 1910.

So why, if you'll excuse the pun, doesn't it all fall down?

"The most important thing with any of these things is that you create a sense of consistency," says Ken Levine, the creative director of developer Irrational Games. "That you make a leap, but some of it is based on science."

The root of that consistency, in this case, is the one thing Levine won't really talk about. "The prime thing that happens in Columbia to set everything in motion is the thing that makes it float," he says. "And there is a scientific explanation. It's not something that would be possible today, but it's connected to everything that's going on in Columbia."

New stuff will happen when you revisit old areas, a bit like in Arkham Asylum.

So we don't know what makes it float, but we do know why it was built. At the turn of the 20th century, America was at the peak of its national optimism. The previous few decades of innovation, discovery and expansion had made her strong, and Columbia was an emblem of that strength.

But it was also designed to export that strength, and following a violent international incident the floating city became a rogue state and disappeared.

You have found it again. Playing as a cynical former Pinkerton agent called Booker DeWitt who thinks he's seen everything, you're charged by a mysterious benefactor with rescuing a girl called Elizabeth. Imprisoned in a tower for 15 years, Elizabeth is practically your opposite - she's no fool, but she is innocent and naive and bursting to embrace the world you've given up on.

Together you try to escape, which is easier said than done, because half the population wants Elizabeth back in the tower and the other half wants her dead.

Levine salutes 'archaeologist' gamers who enjoy probing the recesses of his games for secrets and background.

Like Irrational's other games, the path you tread is largely chosen for you, but the manner in which you tread is your own design. There are projectile weapons like pistols and shotguns to choose between, and also Vigors - special powers that let you lift enemies from the ground or attack them with flocks of angry birds. You can also direct Elizabeth to use her powers to assist, creating cover, escape routes and even allies to fight alongside you by dragging them into your reality through "Tears" in your surroundings.

One change here is to the way some of your passive abilities, called Nostrums - Infinite's equivalent of BioShock's gene tonics - are utilised.

"When we looked back on the first BioShock," says Levine, "one problem we thought it had was that people didn't feel wholly connected to the character because there wasn't permanence in the character-build system, and you could swap anything out at will."