Gas, so you're not disappointed I'll explain slightly. Firstly BioShock is one of the best games I've ever played. I probably shouldn't have said "based on". It's not based on Atlas Shrugged, but BioShock was inspired completely by Ayn Rand.
The story is that Andrew Ryan, a wealthy malcontent, decides to build a utopian society on the bottom of the ocean and people it with only the best and brightest minds he can recruit.
There are none of the morals or laws that "limit" the scientists on the surface world, so genetic engineering and other questionable sciences soon flourish and everyone in Rapture is "splicing", which is what they call the genetic self-improvements. Well, of course the splicing has terrible side effects and Rapture begins to fall apart as people become disenchanted and chaos breaks out everywhere from the renegade splicers who have more or less taken over.
This is when you enter Rapture.

The story is beautiful, though it's pretty creepy at times and it is a FPS (first person shooter) so it helps if you like those types of games.

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"You gotta help us, Doc. We've tried nothin' and we're all out of ideas!"