Once Upon a Time in Mumbai is the story of two underwold dons whose lives enter parallel lives as they struggle to survive in Mumbai`s criminal underworld. Emraan & Ajay in the role of Sultan and Shoaib are the two man in the world that the society is actually feared off.Prachi Desai appear in the role of Mumtaz the love interest of Shoaib. Kangana on the other hand has done lot of research work by studing the yesteryear actresses of both Bollywood and Hollywood to do justice to her role. She appears as Haji Mastan`s girlfriend Sona.
A cop who just attempted suicide narrates the story for the audience who has migrated to Bombay at a very young age and rises rapidly into the world of deceit and infamy. As the law changed the definitions also changed and the smuggler of gold and watches is called as importer now. A break up of Haji Mastan and his protege Dawood is narrated in this movie.