Jack is the playable character in Bioshock, a witness to the events that occur in Rapture. Although no last name was ever given, some fans refer to him as "Jack Ryan", considering the plot twist regarding his parentage.
Very little was known about Jack's past, only that he had a family and lived on a farm. All that came tumbling down when in 1960, a plane where Jack was a passenger crashed in the middle of the Atlantic Ocean. Jack swims to a lighthouse close by, where for the first time (he thinks), enters the underwater world of Rapture.
After coming in contact with Atlas, an underground resistance leader seeking to end Andrew Ryan's so called grasp on the city, he becomes intent on taking the man down, with Atlas always asking Jack to do every task with the pre-cursor "Would you kindly..."
Through many audio diaries, Jack learns the demise of Rapture came down to two major factors: The obsession and constant pursuit of ADAM, a genetic modifier, and the battle for the city between Ryan and Frank Fontaine, a businessman who smuggled banned items into the city under false ventures.
Eventually, after Jack meets Ryan, he learns that he never really had a farm or a family. He was Andrew Ryan's illegitimate son, taken away after his mother was murdered, forced by Frank Fontaine to be part of an experiment in mind control run by the malevolent Dr. Suchong that they hoped would allow them to gain control of the city. Built to be stronger, faster, and more intelligent, he was made into a human weapon. The way Suchong and Fontaine were able to control him was with the phrase "Would you kindly"; if included in the phrasing of a request or suggestion, Jack will literally be incapable of not following that suggestion. We learn this in an audio diary, when Suchong asks Jack to break a puppy's neck, and the child complies despite not wanting to do so. Ryan tells Jack that that they are father and son, and he is truly only two years old, but with the amount of ADAM inside of him, he has the body of an adult. It becomes clear how powerful the mind control was when Ryan forces Jack to murder him with his golf club, dying in a manner of his choosing, with the resurrection Vita-Chamber nearby purposely deactivated so that he will remain dead; as he puts it, "A man chooses, a slave obeys." It's possible, though unlikely, that Ryan told Jack to kill him in hopes that he could somehow break the mind control Fontaine had on him.
Jack also later learns that Atlas was Fontaine all along, someone who pushed Jack along towards his intended use.
Left for dead by the closest thing he had to a father and betrayed by the only friend he ever had, Jack was saved by the guidance of several Little Sisters, girls who were part of Fontaine's orphanage that were used to become hosts for the sea slug that contained ADAM. After teaming up with Brigid Tenenbaum, the creator of ADAM and the Little Sisters, she saves Jack from the mind control that Fontaine had been using against him. Jack eventually takes down Fontaine, and depending on whether the player harvested or saved the Little Sisters, either received a happy ending involving the Sisters to grow up and live happy lives, or where Jack took over Rapture and sent splicers to overrun the surface.