Booker DeWitt is slumped in his office, his numb stupor disturbed by a persistent knocking at the door. Historically for Booker, whatever's on the other side isn't going to be good news. And so it proves in Burial at Sea Episode 1, the first story based DLC for Bioshock Infinite. Its opening minutes follow the beats of a hardboiled detective novel. Moments later, the door swings open to reveal a girl, a case, and a whole ocean of trouble.

It's not that simple, of course. The girl is Elizabeth, the case is to find a missing child, and the ocean belongs to Rapture. If you've played Bioshock Infinite, you've probably got some questions right now. Questions like, "wha-?", "huh?", and "come again?" It's not that these characters couldn't exist in this city - Infinite's ending made sure of that - but it's a surprise to see Booker feeling so at home here, especially because this isn't some intra-dimensional knock-off Rapture. It's the Rapture, the one we knew and shot bees at, shown two years before Andrew Ryan was introduced to the business end of a 9-iron.

Such a setup could easily seem forced, but Burial at Sea is a proper follow-up to Bioshock Infinite's story - just one that happens to expand our perspective on one of the most iconic game worlds of recent years. "You could take any of these cool characters, stick them together, and there's some fan service there," says Burial at Sea's producer Don Roy about the plot, "but we wanted do it in a truly meaningful way, so that it stands on its own and is impactful."

And Burial at Sea is impactful, especially throughout its opening areas, where Booker and Elizabeth explore the commercial district of Rapture seeking an an invite to meet with one of the city's more illustrious residents. Here, the shops and corridors are packed with detail, providing insight into the workings of the Rapture before the fall. "I'm very excited to see if players and fans have that experience," says Roy. "One of the great things about Infinite was the introduction of the life in the world. So the narrative that we could tell through civilians just walking around and being there, so that is a great new tool in our toolbox that we were able to bring to Burial at Sea."

To call it "life" is a bit of a push. As in Infinite, Burial at Sea's Rapture feels more like an explorable stage - its actors dutifully perform their vignettes as you pass. But the hyper-real spectacle does sell the idea of a place where magic has become the norm - from the theatrical flourish of a Houdini splicer serving drinks to his customers, to the awkward silence that follows a line of Little Sisters. It's an effective snapshot of a growing excess that would of course result in Rapture's horrifying implosion.

Nowhere is this more apparent than when meeting Sander Cohen, one of the few returning characters from Bioshock 1. His encounter is the highlight of this short Part 1 campaign, and it effortlessly ties together ideas and plot strands from both Irrational Bioshocks. It also shines a light on Elizabeth's perception of Booker. Their relationship is colder and more formal throughout the story; a result of the fact that Booker doesn't know who Elizabeth is, and that Elizabeth absolutely knows who he is.

"I love it," says Roy of this new incarnation of Elizabeth, "because she's still truly evolving as a character. I'm interested to see how players react to her, because there is a stark difference. Her existence is now coloured by the violence that she's witnessed in Columbia - and partaken in - and the transformation at the end to be who she ultimately became. So she's coming into the situation with a purpose, but she's coloured by all those experiences."

Burial at Sea does switch pace from exploration to combat, which brings back another recurring Bioshock scenario - rooms full of crazed, magical psychopaths in need of killing. Burial at Sea sticks with Bioshock Infinite's combat system, tweaked and squashed to fit inside the rooms and corridors of Rapture. "That's one of those ones where everybody in the room is 'yeah, we know how to do this,'" says Roy, "And then you start building and you're like 'oh wait, we have to build this from the ground up.' But it's great, because you end up with the best work when you do that. Trying to piece together something is never going to be as good as holistically going, 'we're going to make this new, and we're going all in.'

"Having to rebalance and rework the systems so they fit Bioshock 1's structure - because it's Rapture, and it's hallways and it's more enclosed encounters - was a big challenge for the team. But so many great lessons learned, and we flexed muscles we hadn't in a long time. And you start getting it, and you start making good decisions about the amount of resources the player has, and one day you're playing and it's really fun. You're running for your life, and you're having to make hard decisions, but they're fun hard decisions to make."

Not everything transfers cleanly. Rapture's using Infinite's Vigors now, for instance, although the fan-designed Old Man Winter bottle does reintroduce one of Bioshock 1's powers. "That's the best case scenario," explains Roy. "We realise we need a plasmid, we know we'd like to have some functionality that was in Bio 1, and we have this thing show up. It's amazing, and it looks like it could have been done out of our studio, and it's compelling and awesome and when you see that poster you're like, 'I want that bottle'."

There's an attempt to explain the switch: a series of audiologs from Ryan's researcher Suchong, as he investigates the after-effects of Elizabeth's reality hopping. The reasoning behind the inclusion of Infinite's Skyhook, on the other hand is arbitrary and throwaway. Brief justifications aside, the combat system feels great in its new home. Where Infinite could overwhelm through numerous systems in large arenas, Burial at Sea's tighter space and lighter resources mean every element feels essential. You need Elizabeth's tears to buy you cover, Vigor traps to cut off possible flanking routes, and the scarce handful of bullets in your handful of guns. Fights feel scrappy and reactive in a way that Infinite never captured and, for all the grandness of the opening, it's this focus on scavenging, planning and scrambling through that provides the majority of the DLC's thrills.

If there's a downside, it's those thrills are packed into such a small package. At roughly ninety minutes, it's short, even for a DLC campaign, and ends with a cliffhanger to set up next year's Episode 2, in which you'll play as Elizabeth. Of that chapter, Don Roy was understandably cagey. "The one thing I will say is that she's very different from Booker. The exciting thing about doing that - the reason to do that - is to be able to see the world through her eyes. As we were talking through the possibilities, it became, 'well, we can't not do this'. It's what fans want, it's what we want."

Bioshock Infinite: Burial at Sea - Episode 1 is due out on November 12th.