This particular plot twist is one that I definitely didn’t see coming the first time around. In Bioshock you play as Jack as he traverses the underwater city of Rapture. When he first arrives he meets Atlas, a man who begs Jack to help him save his family. Unfortunately for Jack, Atlas isn’t who he says he is.
Atlas is actually Frank Fontaine, a criminal mastermind and arch-nemesis of Andrew Ryan, the man who helped create Rapture. In fact, Jack only arrives in Rapture because he is Frank Fontaine’s last ditch effort to take out Andrew Ryan. One of the biggest plot twists in all of gaming, in this writer’s opinion, is when Jack realises that he was Fontaine’s pawn all along.