Frank Fontaine is the man responsible for the fall of Rapture, having lead the revolution against Andrew Ryan under the guise of "Atlas." From his humble beginnings running Fontaine Fisheries, Frank quickly grew to be a prominent figure in Rapture when he heavily invested in Bridgette Tenenbaum and Dr. Yi Suchong's research of Adam-enriched sea slugs. This led to the founding of Fontaine Futuristics, a biotechnology company that rose to power quickly and drew great concern from Andrew Ryan. Ryan perceived Frank and Fontaine Futuristics as a threat to stability of his underwater city due to its smuggling operations and influence among the citizens of Rapture.

As part of his grand scheme to control Rapture, Fontaine got his hands on Andrew Ryan and Jasmine Jolene's illegitimate son and, with the help of Dr. Suchong, genetically altered the child to become his personal assassin. You eventually discover that you (Jack) are this would-be assassin and have been infused with a mind control plasmid that allows Fontaine/Atlas to command and, to a lesser effect, kill you with specific phrases. Being the resilient genetically altered superhuman that you are, though, you eventually discover a way to clear your mind of Fontaine's influence.

All of this leads to Frank Fontaine being the game's main antagonist and final boss battle. To ensure that he stands a chance against you, Fontaine injects himself with a massive dose of Adam that transforms him from a regular human to a hulking giant with several powerful plasmid powers. After siphoning all of his Adam, you'll have to rely on the help of a small army of Little Sisters to end his life.