So, to summarise: the whole game is an escort quest.
It most certainly is not, or at least not the kind of escort quest you're used to. Levine says Elizabeth is one of the most sophisticated AIs ever created, not simply supporting you in combat or pointing you towards the next objective, but interacting with you and the environment differently depending on where you stand, where you look and what you're doing.
Stand near a souvenir stall and she might breathlessly seize on a tacky statuette, mistaking it for gold. Don't pay attention for a while and when you turn your head she might be dangerously far afield, distracted by a bit of scenery or a distant cry. There's a slew of possible Elizabeth actions per area, and she'll vary them in response to your behaviour.
Irrational wants the girl to compel even when she's standing still, waiting for you to lead on - her animations are the product of meticulous observation, be it hugging her waist worriedly or covering a scream with her hand when Songbird crashes the party (he's beautifully animated too, twitching his head to and fro like a grotesquely inflated sparrow). You might be forced to keep her company, but you'll find she's company worth keeping.
If she's the tactical support, what does Booker bring to Bioshock Infinite combat?
His basic capabilities are similar to those of previous Bioshock protagonists - a mixture of conventional firepower and genetic superpowers, now known as Vigors and Nostrums. Vigors are active abilities you'll fire from your left hand, and they include Bioshock's spin on the Jedi Force lift, briefly trapping dug-in foes in free-fall, and the Murder of Crows, delivering crowd control in the form of a hundred angry avians. Nostrums are permanent upgrades that bestow passive benefits, like the self-explanatory Spring-Heeled and pugilist's favourite Executioner. One - Pot Luck - asks you to choose between three or four different upgrades.
The guns are brilliant little slabs of steampunk, ranging from a fat-barrelled machine gun - the Rolston Reciprocating Repeater - to a rocket launcher you reload by winding a crank. There's what looks like a Luger pistol, too, plus an old school bolt-action sniper rifle. But the true joy of Bioshock Infinite combat stems not from the weapons you wield, as the way you get around, and the way the game's sprawling, towering levels turn on this.
Fingers crossed for a jetpack.
Sorry, but we think you'll like the Skyrails just as much. Not long after reaching Columbia, Booker acquires a wrist-mounted device that lets you latch onto Columbia's single-seater rollercoasters at will. Doing so will be your salvation during pitched battles, when the odds abruptly swell to include snipers, artillery turrets, flare guns shaped like pipe organs that summon blimps, and hordes of yelling redcoats. Booker's mobility under pressure stands comparison with Bionic Commando - you can leap between Skylines at intersections, or snap onto them after dropping from airship hatchways.
What's Elizabeth doing while you're zipping around?
Either keeping her head down - she's reasonably self-sufficient - or following close behind, using a Skyline hook of her own. She's a good person to have at your back, thanks to the aforesaid paranormal abilities, transforming carefully placed patches of juddery dimension-distortion into ammo stockpiles, turrets and even escape routes.
Fold in Vigors and Nostrums, and we're talking firefights that rival Crysis and Halo for density of options. It's pretty thrilling.
So is combat the game's strongest feature?
Actually, the thing we're most interested in is the world itself. One expects an arresting backdrop from Irrational and they haven't disappointed, taking Rapture's organic detail and slathering it across a much wider (and more heavily populated) area. It's no Grand Theft Auto, mind. "I don't want people to think this is an open world, because it isn't, but it's hubs and spokes on a much larger scale than before," Levine told Eurogamer last year. "There's a huge amount of verticality, there's a lot of exploration, a lot of very large outdoor spaces, and then very traditional BioShock-style interiors."