Inspired by true events, as claimed by the makers, Mumbai Saga begins with a dreaded gangster Amartya Rao (John Abraham) gunning down a businessman (Samir Soni) in broad daylight. The film rewinds eight years back where we are introduced to his younger days when he earned his livelihood by selling vegetables at the railway station. His younger brother Arjun is the apple of his eye.
When the lad is hospitalized after a spat with one of Gaitonde's (Amole Gupte) henchmen over extortion, Amartya lets his brawn do all the talking and lands up in prison. With the help of a powerful political figure Bhau (Mahesh Manjrekar), Anna (Suniel Shetty in a cameo) and a drug dealer Naari Khan (Gulshan Grover) whom Amartya befriends during his stint in the jail, the latter rises to become one of the most powerful gangsters in the city. "Bandook toh sirf shaunk ke liye rakhta hoon, darane ke liye naam hi kaafi hai," Amartya proudly declares to scare an opponent.
However, his existence gets threatened when the slain businessman's wife (Anjana Sukhani) announces a bounty of Rs 10 crore to whoever slays him, which sets a tough cop, Vijay Savarkar (Emraan Hashmi) in pursuit of Amartya.