The movie starts with D. N. Tripathi (Piyush Mishra), a lyricist, who writes a biography on Sanjay "Sanju" Dutt, (Ranbir Kapoor), comparing him with Mahatma Gandhi. A flabbergasted Sanjay has him thrown out. The Bombay High Court delivers its verdict regarding the 1993 Bombay bombings and sentences Dutt to five years jail term for violating the Arms Act, 1959. His wife Manyata Dutt (Dia Mirza) approaches Winnie Diaz, (Anushka Sharma) a London-based writer, to write Sanjay's biography and present his version of his life to the public. Although unwilling at first, Winnie is asked by Zubin Mistry (Jim Sarbh), a real estate builder, to not write the biography, which intrigues her. Winnie first interviews Sanjay, and his life is revealed in a flashback.
Sanjay's father Sunil Dutt (Paresh Rawal) plans to launch his acting career in Bollywood with the film Rocky (1981). Upset over his father's controlling behaviour on set, Sanjay is encouraged by his friend Zubin "God" Mistry to try drugs for the first time. Sanjay soon discovers that his mother Nargis (Manisha Koirala) is suffering from cancer and is taken to New York for treatment. This incident furthers his descent into alcoholism and drug addiction. He meets Kamlesh (Vicky Kaushal), a fan of Nargis, in New York, and they become quick friends. Kamlesh manages to make Sanjay stay his addiction; after learning of his girlfriend Ruby's (Sonam Kapoor) fixed marriage due to his ongoing drug addiction, Sanjay relapses. Kamli convinces Ruby to register marry Sanjay; upset with his intoxicated behaviour, she leaves him. Nargis passes away three days before the release of Rocky, which takes an emotional toll on Sanjay. Sanjay agrees to attend a rehabilitation centre in the United States, and eventually recovers with the help of his father and Kamlesh. On returning to India, he meets with God and beats him for selling him drugs.
The later part of Dutt's life is narrated by his now-estranged friend Kamlesh, who Winnie tracks down. In the 1990s, Sanjay turns to bodybuilding and his career in Bollywood has greatly improved. After the 1992 Babri Masjid demolition, Sanjay acquires three AK-56 rifles to ensure the protection of his father, who had by then ventured into politics, and sister, Priya Dutt. A series of bombings occur in Bombay shortly thereafter. Sanjay is arrested in 1993 for possession of illegal arms supposedly supplied by the D-Company to be trafficked for protection during potential communal rioting after the bombings. Sanjay is convicted and sentenced to imprisonment for five years under Terrorist and Disruptive Activities (Prevention) Act (TADA), threatening his career. Indian media labels him as a terrorist, further tarnishing his public image. Believing Sanjay to be guilty, Kamlesh severs ties with him.
After his release in 1995, Sanjay appears in a string of flops. When his popularity had begun to wane, he is signed for the title role in Munna Bhai M.B.B.S. (2003), co-starring his father. Sanjay's performance garners him the Filmfare Award for Best Performance in a Comic Role and his public image improves, making his father proud. Shortly afterwards, his father passes away. In 2006, the Bombay High Court finds Dutt guilty under the Possession of Arms Act, while not finding him to be a terrorist. In 2013, he is arrested again by the same verdict upheld by the Supreme Court of India. Sanjay convinces Winnie and Kamlesh of his innocence by blaming the media for falsely accusing him of terrorism, and is subsequently released from prison in 2016. Sanjay finds out that Winnie has completed writing his biography, titled Kuch Toh Log Kahenge (translation: People will keep saying something), named after one of his father's favourite songs from the film Amar Prem (1972). He exits the jail and reunites with Kamlesh and Manyata. (wikipedia)