Once Upon A Time In Mumbaai (also known as Mumbai Goes Rewind) is a forthcoming Bollywood film, directed by Milan Luthria and starring Ajay Devgan, Emraan Hashmi, Prachi Desai, and Kangana Ranaut. The film is produced by Balaji Telefilms and Popcorn Entertainment. Once Upon A Time In Mumbaai is scheduled for release on 30 July 2010.

The film will cover the entire growth of the Mumbai underworld, from crime and smuggling in its early stages through its connection with international terrorism in recent times. It is based on actual happenings. It is believed to be loosely based on the lives of Haji Mastan and Dawood Ibrahim.
Seen through the eyes of a police officer, Once Upon A Time in Mumbaai traces the changing face of the Mumbai underworld.
The film, set primarily in 1970-Mumbai, follows the rise of Sultan (Ajay Devgn), and his eventual fall, when his protg Shoaib (Emraan Hashmi), challenges his supremacy, and usurps power to rule the murky underbelly of Mumbai.