Plot

Gandhi Mahaan, a school teacher, is abandoned by his family after he decides to live a life of his own, with personal freedom.

It is a narrative of a series of events that transform the whole life of an ordinary man as well as all the people around him. Mahaan features real-life father-son duo Vikram and Dhruv Vikram together for the very first time, along with Bobby Simha and Simran in pivotal roles. It is the story of a man whose family leaves him when he strays from the path of ideological living in his search for personal freedom. However, as he realizes his ambitions, he also misses the presence of his son in his life. Having fulfilled his dream of becoming a billionaire, does life give him a second chance to be a father? This story is about how his life goes through an unexpected series of events in this thrilling, action packed journey.

1968: Mohandoss is a middle-aged liquor-ban activist whose son Gandhi Mahaan is the exact opposite of him, and is addicted to gambling and drinking. Sathyavan Soosaiyappan is his friend, who gambles against Gnanodhayam. Gandhi is a prodigy in gambling and always helps Sathya win matches. One time, Sathya keeps on losing and confronts Gandhi, who tells that he has no choice but to make him lose because Gnanam kidnapped his dog, Joker. Everyone fights over it, and their parents had to interfere in it. Mohandoss makes Gandhi promise that he will lead the liquor ban movement and live as a Mahaan.

1996: Gandhi is a 40-year-old commerce teacher at the local government school. He is married to Nachi and has a son Dada. On his birthday, Gandhi leaves for a temple where he meets a beggar, who tells that Gandhi leads a life like 95% of people with morals or principals, but cannot lead a life like 5% of people, who do not follow morals or principles in life. At night, Gandhi remembers the beggar's words where Nachi tells Gandhi that she and her friends are going on a trip to Tirupati. Gandhi uses this as an opportunity to live life to the fullest and goes to a bar, where he meets one of his old students, Rakesh Christopher alias Rocky, who decides to help Gandhi fulfill his dreams.

Gandhi is then taken to Rocky's father Sathya, where Gandhi plays rummy and wins a lot of money. Sathya recognizes Gandhi as his childhood friend due to the scar. The next morning, Gandhi reaches back home, where Nachi deduces that Gandhi is in a drunken state and leaves him, taking Dada with her, since Gandhi broke the rule of never drinking. Gandhi goes to Nachi's house to try a reconciliation, but is ousted where he decides to live with Sathya and Rocky, and they all come up with selling the liquor that Sathya's father Soosaiyappan made and names the brand as Sooraa.

1998: Gandhi, Sathya and Rocky are all rich from selling the liquor and plan to start a syndicate where only their drink could be sold in the bars in Tamil Nadu. One of the members opposes and attacks them, but Gandhi fights all of them off. This incident causes Sathya to get paranoid and suggests that they stop selling liquor.

2003: Gandhi, Sathya and Rocky have become influential people in society. However, The Tamil Nadu Government announces that only approved liquor is allowed to be sold in bars, and with this development, they decide to visit one of the party members, Gnanam, whom they then deduce that he is also one of their childhood friends and agree to a deal that Sooraa can be sold in bars.

2016: The gang is running a liquor mafia in Tamil Nadu, and also the CBI is trying to find evidence to arrest them. At Rocky's wedding, Gnanam proposes a deal to Gandhi that he can let them produce the entire stock of the liquor in Tamil Nadu by killing off the DC, which they do so. Gnanam then meets Sathya and Gandhi and proposes that they produce all the lower quality liquor under different brands, which Sathya acts grotesquely and Gandhi admits that they killed off the collector. With all of this, Gnanam decides to cancel the license of Sooraa, but they learn that Gnanam had illicitly had a son with a woman in his old colony and use the information to blackmail Gnanam into reinstating the license of Sooraa. Gandhi leaves to a festival in which one of his gang members, Michael, invites him.

In the middle of the festival, a stranger arrives and reveals himself to Gandhi as his son Dada. In this timeframe, Dada kills Michael and tells Gandhi that he was appointed on a special mission to get rid of the liquor mafia. However, Dada ensures Gandhi that he will not kill him, but he will kill everyone else.

It is revealed that Gnanam wanted to appoint the team to seek revenge on the entire group. Gandhi visits Gnanam and tells him to stop the operation before it gets worse. Dada takes Anthony and kills him. Enraged, Rocky tries to find out who killed Anthony and Michael, where Gandhi is forced to reveal to Rocky that the officer that killed them both was his son and he cannot do anything about it. Hearing this, Rocky tries to convince Dada as an older brother. Dada decides to kill Rocky but is heavily injured in the process by Gandhi.

Unable to bear Rocky's death, Gandhi decides not to reveal it to Sathya. He visits Nachi at her new home, and both reconcile and get Nachi to convince Dada, but to no avail. She then moves into Gandhi's house and lives there, but in a sudden turn of events, Dada is kidnapped by Gnanam because Gnanam learnt that Dada is Gandhi's son and that he will only release Dada if Gandhi kills Sathya. Gandhi meets Sathya, where Sathya has a video of Dada killing Rocky and he sees that Gandhi is also present in the video. Gandhi tries to explain to Sathya that Dada is his son and reveals the whole incident. An enraged Sathya does not listen and tries to get his men to kill Gandhi, but Gandhi ends up killing all of them and Sathya in the process. Gandhi leaves to a remote location and burns the car that he and Sathya first bought together. Dada arrives and picks up Gandhi, revealing that he was able to escape from Gnanam and tells that it was his entire idea and that he wanted Gandhi to kill Sathya so he sent the video footage to him.

Enraged, Gandhi insults Dada and Nachi, to which Dada reacts grotesquely to the remarks. Later, Gandhi forms a plan and tells that Nachi was also kidnapped by Gnanam and gets Dada to visit Gnanam and kill him. It is revealed that Nachi was not kidnapped and that she is given a bhang laddu by Manickam to put her into deep sleep. Dada visits Gnanam and kills him, while on the phone with Gandhi. Gandhi reveals that he has destroyed all the liquor in his house and signed all of his wealth to an NGO that helps people recover from addiction and shut down all the factories. However, Gandhi also tells Dada that Nachi was never kidnapped, but that he used this opportunity to get Dada arrested and teach him that: in life, everything has to be balanced and there should not be an extreme of anything. With that, Gandhi tells him that he has become a Mahaan.