Once Upon a Time in Mumbaai is a 2010 Indian Hindi-language neo-noir crime thriller film written by Rajat Arora and directed by Milan Luthria. It stars Ajay Devgn, Emraan Hashmi, Kangana Ranaut, Prachi Desai and Randeep Hooda. Produced by Ekta Kapoor under the Balaji Motion Pictures banner, Once Upon A Time in Mumbaai was released on 30 July 2010 to generally positive reviews from critics, and was a commercial success. The film is loosely based on the lives of Mumbai underworld gangsters Haji Mastan and Dawood Ibrahim. It was made on a budget of ₹38 crore (₹28 crore for production and ₹10 crore for publicity and advertising). A sequel, Once Upon a Time in Mumbai Dobaara!, was released in 2013.

The film opens in March 1993 in Mumbai, with a suicide attempt by DCP Agnel Wilson on the pretext of the recent Mumbai bombings. When questioned by his superior, Inspector General Aditya Mandke, over his actions, he breaks down and claims that the bombings are his own fault. Wilson then recounts that 18 years ago, when he was posted as the ACP in the Mumbai crime branch, his inability to take necessary action led to the rise of Shoaib Khan, a dreaded gangster who played a central role in the bombings.

Throughout the film, Wilson narrates the story of 1970s Mumbai, when it was ruled by a kind-hearted smuggler Sultan Mirza, and how Mirza's eventual downfall led to Shoaib's rise to power.

After experiencing a flood in his hometown of Madras, a young Mirza arrives in Mumbai, where he lands a job as a coal shoveller near the docks. Despite his modest earnings, Mirza consistently aids the poor and needy, earning their respect and admiration. As a result, Mirza has been given the nickname Sultan.

As a grown man, Sultan Mirza becomes the kingpin of Mumbai's smuggling underworld in the '70s. Through his influence, Mirza peacefully divides the city among four gangsters, thus thwarting police efforts to curb illegal activities. Despite being a criminal, Sultan Mirza is portrayed as a man of principle with a heart of gold and a godfather-like figure to the people, even going to the extent of refraining from smuggling specific contraband like drugs which was against his Muslim faith.

Meanwhile, Shoaib, even in his childhood was a very ambitious person with a dark and daring character, who was frequently involved in petty theft. His father, Hussain Khan, who is a Sub-Inspector with the Mumbai Police, tries in vain to guide and control his son; his anger against Shoaib began years ago when Shoaib and his best friend Javed were stealing money and got caught red handed by a man, by teaching his son a lesson, Hussain slapped him 5 times. Hussain locks Shoaib in jail, but Shoaib angers him as both Shoaib and Wilson make a deal saying that Shoaib wants to follow another path. Worried, Hussain turns to Sultan for help. Sultan agrees and helps the young man set up an electronics shop. But Shoaib is unsatisfied, as his only real ambition is to become rich and powerful, like Sultan, who is his idol.

Mirza has a crush on actress Rehana and the two eventually begin dating. Sultan invests black money in her upcoming films. Wilson moves to stop Rehana's films funded by Sultan. Later, Sultan and Rehana frame Wilson to make it look as if Wilson is accepting a bribe, which damages his credibility.

Shoaib's beautiful girlfriend, Mumtaz, works in a local jewellery shop, which Shoaib visits often, to the aggravation of her boss. Shoaib gives her a beautiful necklace, which, unbeknownst to Mumtaz, Shoaib had stolen from a lady during a home robbery. Later, that lady comes to the shop with her husband to buy more jewellery. The lady soon recognizes her own necklace being worn by Mumtaz and accuses her of allegedly stealing her jewellery and publicly insults her. Following this, Mumtaz admits to the outraged customer that her boyfriend had given the item to her. The lady and her husband demand she takes them to her boyfriend's shop, where they confront him. This enrages Shoaib, who beats up the husband and destroys his own shop.