You are 11 years old and you live with your parents in Rapture, an underwater city built by businesswoman Andrea Ryan which was meant a haven for scientists and artists to use their talents to the fullest and not be held back by Society.

She is a true believer in objectivism the belief that opportunity to use reason without the initiation of force is necessary to achieve moral values, each individual has an inalienable moral right to act as his judgment directs and to keep the product of his effort. The scientists in Rapture found a substance dubbed Adam in sea slugs that can give the user unnatural powers. And to get more of this drug they started conditioning kids to gather the Adam with spliced-up men wearing diving suits as protectors, they named Big Daddies.

You are the last child not to be conditioned and every other child is going around with their Big Daddy gathering Adam from corpses you hide around hoping not to be caught and turned into one of those little Frankensteins. But right now you are wandering around for Frolic for food, but that's when a creature appears. It looked like a Big Daddy, but with a more feminine physique. This is a Big Mommy. Its diving suit looks a little different than usual and they looked taller and much more thin, and their helmet had a large sphere with two portholes, it turns it's helmet while looking at you.