The film begins during the historic independence of Bangladesh, where two abandoned boys, Bikram and Bala, are discovered by a man named Lateef. He offers them a chance to work for him in an illicit operation involving gun smuggling. Lateef becomes a guardian to the boys, who are innocent yet vulnerable. However, their lives take a dark turn when an army officer demands that Lateef leave one boy behind to serve as his child sex slave, threatening his daughter if he refuses. In a heroic act, Bikram bravely decides to sacrifice himself to save Bala, yet Bala, determined to save his friend, returns and kills the officer. Tragically, this leads to Lateef's demise at the hands of the police.
The boys manage to escape to Kolkata by train, where they begin to earn a living working at a small restaurant. Tensions rise when Bala confronts the owner, who humiliates him by calling him a refugee. This encounter sparks a desperate act, as they start looting trains of coal and selling it cheaply in the market. As they grow older, Bikram (played by [Ranveer Singh]) and Bala (played by [Arjun Kapoor]) escalate their operations from petty thieves to notorious train robbers, even taking down a famous coal bandit named Divakar, who scorns them as "refugees." But despite their criminal activities, they long for an identity within India.
In a twist of fate, they transition from lawbreakers to legitimate businessmen, thanks to the guidance of Kali Kaka ([Saurabh Shukla]), their lawyer. They use their resources to support those in need, effectively transforming into respectable figures—dubbed the 'Gunday.' However, their newfound status comes with dangers, and A.C.P. Satyajeet Sarkar ([Irrfan Khan]) is sent to Kolkata with a firm warning: “I will arrest you the day I find a single piece of evidence.”
When Nandita ([Priyanka Chopra]), a captivating cabaret dancer, enters their lives, both friends find themselves infatuated with her. They agree that the one she loves will be her future husband. After several attempts to win her over, Nandita invites them to a theater. A confrontation ensues when Bala reacts violently to an insult directed at Nandita, leading him to kill a man. To ensure his safety, Bikram urges Bala to leave for a while and vows not to see Nandita until his return. However, Nandita reveals her feelings for Bikram and insists he attend the Durga Puja, claiming she won't meet him again if he doesn't.
Bikram ultimately succumbs to his feelings and breaks his promise to Bala. Upon learning this betrayal through his henchman Himanshu ([Anant Sharma]), Bala returns during the festivities, where he confronts his friend. A heated exchange culminates in a shooting incident where Bala's rage inadvertently injures Nandita, escalating the rift between the two boys who were once inseparable.
Following this, Bala discovers that all their enterprises are funded through Bikram's name, leading to a heated argument over their business division. They agree to split it, but tensions remain high as Bala provocatively mentions sharing Nandita as well. In a fit of anger, Bikram prevails in their ensuing fight, yet chooses to show mercy due to their shared history. Their strained friendships worsen when Nandita gets kidnapped by Bala, forcing Bikram to ally with Sarkar against him.
Yet, Bala realizes his mistakes and allows Nandita to go, unaware that she has been secretly working with Sarkar to apprehend both men. Meanwhile, Bikram uncovers the truth about Nandita, as well as the fact that the man Bala thought he killed is, in fact, alive and working for the police.
As tensions reach a boiling point, Bala finds himself betrayed by his henchmen, who are seeking revenge for the death of Divakar, their brother. In a last-minute attempt, Bikram rescues Bala, and they join forces against their common enemies. However, their triumph is short-lived as they soon face a police force led by Sarkar and Nandita. In a gripping chase, they find themselves cornered, where Nandita professes her love for Bikram, urging him to surrender. Just then, an opportunity arises when they spot a passing train, and both Bikram and Bala make a daring escape attempt, with bullets flying around them as the police close in.
