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He's talking, of course, about the link between starlet Elizabeth and the towering Songbird who's both kept her captive and kept her company for most of her life. The Songbird is her jailor and her protector, and won't let anybody come between it and her - even you.

"I was talking to a friend of mine," explains Levine, "and he said the Big Daddy was a god of the ocean and so the Songbird needs to be a god of the sky. He has to be huge and overwhelming, and this 30ft tall thing has to have an intense, strange, weird relationship with Elizabeth. And that's what we want to do. Relationships are messy and I don't just mean romantic relationships: just people's relationships."

SHOP TALK
Our walkthrough begins with Elizabeth and Booker picking their way through a deserted Major Notion's Sundries and Novelties shop. The first thing that hits us isn't the sunlight streaming in through the windows (although it's a welcome sight after Rapture's total dependence on electric-powered illumination) but Booker DeWitt's voice.

It's odd to hear a Bioshock protagonist speaking outside of an intro video, let alone engaging in a full-on conversation with a partner who's persistently on screen.

As Booker and Elizabeth weave through the shop they're always conversing. Sometimes Elizabeth will be joking about - parading around with an Abraham Lincoln mask on while reciting the Gettysburg Address - and Booker will be chuckling; other times she'll be marvelling at the USA paraphernalia on show and Booker will simply be calming her down.

If Booker's not close enough to Elizabeth for these moments to kick in they won't happen, but that's okay - you won't necessarily miss out. When the pair are in close enough proximity to one another the next time she passes by the items needed to trigger these unique reactions, even if it's not until hours later into their journey, they'll happen then.

Much later on when the duo are caught up in a major battle they're still talking to one another - about combat tactics this time - but for now the chit-chat is jovial as Booker ransacks the shop for gear and cash: pinching dollars from the till, a Vigor from a shelf and a Nostrum from a barrel. As Ken says, Irrational are really playing on the relationships.

Vigors and Nostrums are Infinite's equivalent of Plasmids and Tonics. Booker snaffles a Bucking Bronco Vigor that lets him pull objects and people up into the air to either shoot them or force-push them away, and a Pot Luck Nostrum that lets him choose between a Spring-Heeled movement speed increase power, a Winter's Shield Sky-Line bonus or an Executioner melée upgrade. The selection's handled with the face buttons, and at no point does he need to enter a menu system to equip them.

THUNDER BIRD
The ransacking is soon cut short by an interruption from none other than the Songbird. It lands outside the store windows with a crash that upends half the knick-knacks in the shop, and its green eyes turn a threatening shade of orange as it scans the insides of the building looking for life. Elizabeth ducks behind a counter and stifles a scream.

Booker quickly crouches by her side and out of sight. When the bird gives up looking and leaves in search of other hidey-holes, Elizabeth breathlessly explains how she'd rather die than fall back into its capture before daring to venture back outside.

Moving out of the shop and into the streets, Elizabeth and Booker spy Comstock House in the distance (a Cinderella's Castle-like building home to the man who can supposedly help Elizabeth control her timefiddling powers) and press on towards it. Their path takes them through hostile Vox Populi territory, and here's where their plan begins to fall apart.

Right from the word go our duo attracts unwanted attention. They're clearly out of place and although rowdy individuals can be frightened off when Booker aims his gun at them (simply squeezing l will do that job nicely) the deeper Booker and Elizabeth push into Vox territory, the less welcoming the group becomes.

Booker passes up the opportunity to euthanise an injured horse when our demo's pilot declines to press a during a button prompt. When he finds a lynch mob preparing to execute a postman, however, the button prompt appears and we see Booker interrupt and save the innocent's life.

The mob wants blood and Booker's will do, and so the mother of all fights begins. Booker's first priority is to mow down a man winding up a flashing contraption with bellows and speakers to prevent an alarm from sounding, after which he equips a shotgun and fires at another Vox Populi fighter: splashing red paint all over the wall behind.

From there he legs it across an open square and into the Sky-Line station, ready to gun down the fighters using the twisting tracks to flank him.

DUEL-WIELDING
Unlike previous Bioshocks, Infinite uses Halo's two-weapon system. Booker discards his current weapon whenever he grabs a new one, making combat a far more dynamic experience than in previous iterations. Combined with the thrill of leaping on and off and in between Sky-Lines, having to drop and scoop up extra weapons injects added urgency to the fighting.

There's certainly a lot to wrap your head around at any one time - including the small matter of asking Elizabeth to warp cover into existence when you most need it - and so it comes as no surprise when Booker looks back down to the alarm kart from afar and spies another combatant winding up the machine. Unfortunately there's no time to react and, moments later, a flare shoots up into the sky, signalling for backup.

Triggering these alarms in Rapture would normally cause a handful of hoverbots to enter the room and start gunning in your direction - a formidable challenge most Bioshock players are overly familiar with - but Infinite's reinforcements are far more daunting.

Forget tiny robots: when the alarm sounds a giant Zeppelin breaks through the cloud cover to drop off fresh troops and then shell the city from high above. It's a good job Infinite's introduced a regenerative health system to help cope with the extra influx of firepower.

Despite their pomp and size, Zeppelins aren't boss battles. They're simply your punishment for failing to nix alarms in time, and can be taken down in a variety of ways. Singling out the rocket-firing Vox Populi, depriving them of their weapons and firing them upwards is one way of tackling the Zeppelin, albeit a very slow method. Every lethal blimp's health bar is gigantic so this tactic demands perseverance to see it through.

DREAD ZEPPELIN
Alternatively, Elizabeth can bring fixed rocket launcher emplacements through tears and into Columbia's streets for Booker to unleash high-speed volleys of explosives in the airship's direction. The act demands a lot from Elizabeth, however, and if Booker asked her to bring cover into the world earlier on she won't have enough juice in the tank to create the launcher.

In the end, neither option interests Irrational's demo player. Instead he picks plan C and uses Booker's Sky- Hook to jump on the Sky-Lines and move upwards. The Zeppelin is constantly circling the city block, but with some expert planning Booker's able to time his journey so that a peaking Sky-Line brings him to the side of the Zeppelin at an angle that lets him board.

There's a small amount of resistance on board, but not enough to stop him from targeting the glowing engine and destroying the structure with one well-placed shot from the inside before diving out and back onto the Sky-Lines.

Similarities to Master Chief's epic Scarab battles in Halo 3 are undeniable, but there is one distinction: while the Scarab fights were Halo's most epic moments, the Zeppelin scraps in Bioshock Infinite are incidental encounters. Unbelievably, Irrational has even bigger surprises up its sleeve, and they're only hinted at when the Songbird snatches up Elizabeth mere minutes later and flies away with its prize.

Booker's response? To stand up, leap off the side of Columbia and dive down onto a broken Sky-Line track, ready to give chase and reclaim his companion...