Rapture itself is stunning. Established after WWII, it's both of this world and not - a broken civilization in self-imposed exile whose culture has taken a left-turn from the rest of the planet. Each part of the world is packed with references to what was once, and what could have been. "I'm a big believer in creating places that are believable for what they are," says Ken. "I want to make a world where I can push on all the edges and it doesn't break. It feels real. I played a lot of games where it's supposed to be a modern day city but they put barricades at the end of the street and it's like, 'why can't I jump over this barricade?' In Rapture, it feels complete. I can go to the places I'd want to go to, I can experience things in Rapture that I'd want to experience. I don't feel limited and I don't think it's a cheat."
Founded by a visionary named Andrew Ryan, Rapture was designed to be a Capitalist haven where artists and scientists were truly free to express themselves without restraint. A city where the worker earns the "sweat of his brow," where public opinion and governments wouldn't constrain artistic expression and scientific research. But then inevitably it all goes, shall we say, to shit.
The discovery of Adam by a female geneticist called Tenenbaum exacerbates a schism under the sea. Adam comes from a newly-discovered sea slug, which latches onto humans and injects them with stem cells. Adam can be harnessed to produce plasmids, genetic upgrades that can cure disease. They can make you stronger. They can evolve the human body. And when civil war breaks out, control over Adam inevitably becomes crucial. But its effects also make people more dangerous. More powerful. More violent.
Ryan wins out but his dream lies in tatters and Rapture's citizens are warped beyond belief by Adam over-consumption, with side-effects creating hideous physical mutations and psychological damage. And its population is now broadly split in two; you're either abiologically perverted freak working for Ryan or you're scratching out an existence in hiding, unable to escape the city of Rapture and trying to avoid the slavery imposed by its absolutely demented despot.