When fights do break out, however, they're big. Flaming artillery strikes arc through the sky from one island to the next. We use telekinesis to stop a shell from a steam-cannon in mid-air and send it back where it came from. Zipping down a sky line at a rate of knots using a grappling hook, we use a wrench to smack an enemy coming the other way - frock coat flapping in the wind, Derby hat staying firm - off and into a sheer wall. We grab an enemy's shotgun off him with telekinesis, turn it on him and shoot it while it hangs in mid air.
Things get even more dramatic when we're joined by Elizabeth, big-eyed, dark-haired and buxom in her Edwardian frock. A mob of unruly locals attacks - enemies come in larger numbers than they ever did in BioShock - and she summons a howling wind and darkening storm clouds which, combined with DeWitt's electro-shock power, fries them all at once. At another point, she telekinetically moulds a heap of scrap metal into a molten boulder that DeWitt can then hurl at their assailants. The pair talk a lot, DeWitt and Elizabeth clearly signposting targets and strategies for the player's benefit, although Levine says you can always ignore them to pursue your own path.
Levine (in our interview) says that the power development in Infinite will work similarly to the first game's - although we don't hear anything about an Adam equivalent, not yet at any rate - but your options will be expanded, primarily, by the interaction with Elizabeth and by the scope of Columbia's environments. It's maybe not as quite open as it looks - Columbia being broken up into discrete floating chunks keeps your immediate surroundings tight - but there are still much longer sight-lines, with DeWitt using the sniper rifle to pick out enemies on neighbouring 'islands'.
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The demo climaxes with the pair being attacked by a man-machine, a Frankenstein's monster of robot with a heart beating behind glass in his chunky metal torso and a pallid, giant human head with a waxed handlebar moustache. He's not as sinister as a Big Daddy - like a lot about Infinite, there's actually something vaguely comical about him - but he is similarly poignant, and impressively tough.
DeWitt and Elizabeth take him down by bringing down the sky-bridge he's standing on, only for a second, much more terrifying threat to appear: a gigantic winged gryphon, leaping from building to building, a black silhouette even in the sunlight, except for one telling detail: his eyes are glowing, barred, bathyscape portholes, exactly like a Big Daddy's helmet.
It's the only visual reference to BioShock in the entire demo; with the playful glimpse of a toy figure in a diving suit at the start of the trailer, it bookends Irrational's unveiling of the game. That's an artful touch, but in truth, BioShock Infinte doesn't need to lean on its heritage even as lightly as this. The demo may be somewhat theatrical, and the questions stacked far higher than the answers, but Infinite very obviously possesses the intellect, intensity, craftsmanship, playfulness and wonder of its predecessor without seeking to repeat it. If only all sequels could say as much.
BioShock Infinite is planned for release in 2012 on PC, Xbox 360 and PlayStation 3.