In 1986, Rayappan, a Tamil Nadu MLA of the ruling party, is delivering a speech at J.K. Nagar, an impoverished suburb in Chennai, where one of the attendants, a pregnant woman, goes into labour. As a publicity stunt for creating sympathy, Rayappan orders his men to help the woman by lending their dhotis. The woman gives birth to a boy, whom Rayappan christens as Singaravelan, or "Singam" on the stage. Singam grows up as an orphan as his mother had died shortly after the birth of his younger sister, Manimegalai, followed by the death of his father due to overdrinking. Singam's relationship with Manimegalai is quite fraught and strained as the former blames her for their parents death and the latter disapproves of the former's obsession towards Rayappan, for whom he even sold her mother's only nosepiece for party promotions. This led them to not be on talking terms for 20 years.In 2021, Rayappan, now a powerful and influential politician, begins scouting for a suitable candidate to work as a councillor for J.K. Nagar. Vying for attention, Singam strives to seek Rayappan's consideration by performing a media stunt by drinking poison in front of press during Rayappan's arrest and gains his trust, much to the irritation and dismay of Mangalam, Rayappan's right-hand man, who is also vying for the position. Singam manages to emerge as Rayappan's favorite within a short span of time by employing hilarious techniques and his friend Vasu's ideas. Frustrated at being sidelined, an infuriated Mangalam brandishes a bottle at Singam's head; he is subsequently expelled from the ruling party for his ruthless behavior.Shortly after the incident, Singam develops an alter ego, presumably from his injuries who does quite the opposite of Singam's doings. The next day he gets nominated for the counsellor chair, but beforehand, Rayappan asks for 50 lakhs as a deposit for his post, which he got from duping Kamatchi, a wealthy daughter of a moneylending businessman who Singam initially kidnapped with Vasu as a pawn for extorting money from her father with the help of Vasu's hired goons, only to be saved by his alter ego who first resurfaces by thrashing the goons and saving her. During elections, both Singam and Mangalam try various corrupt tricks to lure the voters and increase their vote bank. Initially, Singam is lagging behind in public opinion and votes due to the lack of money power but manages to gamble on the sympathy by intentionally carrying a pregnant woman Shanti in front of the people to gain voters recognition and trust of the people, much to the horror and dismay of Vasu, but Singam ultimately beats Mangalam to become the new counsellor of J.K Nagar.Now enters Damayanthi and Mayor into the fray, who run an illegal construction and money laundering firm for corporate companies under the pretext of NGO named Aashirvaad Foundation. They initially sign a contract with Singam under Rayappan as a mediator for evacuation and demolition of J.K Nagar and offer 50 crores as advance as a bribe. But in the wake of the night, Singam's alter ego again resurfaces and leaks the document to media framing Rayappan, Singam and the ruling party under corruption case. An agitated Rayappan calls for an urgent party meeting to catch the culprit but is shocked to see the vigilante masked in CCTV. He then orders Singam to check the whereabouts of the money hideout, only to find the mask used by the vigilante in there. Now begins the cat and mouse game between Singam and vigilante Singam who flunks out what corrupt Singam's plans to do using various tactics and even causes a distrust with Vasu, who leaves him after seeing his evil intentions and Rayappan questioning his loyalty.Finally, after getting convinced by the evidence found against Singam, Rayappan deduces that Singam is the masked vigilante who has conked all his wrongdoings and orders his henchmen to abduct him, only to be caught red-handed by the income tax department while exchanging money due to the intelligence of the vigilante Singam. After investigations, it is found that Singam handed over only 1/5th of the money. After being bailed out by Kamatchi, who comes across the truth about him and breaks up with him angrily for playing with her emotions. Meanwhile, an angry and denounced Rayappan orders his henchmen to kill Singam in his own area as a statement for betrayal, only to be saved by Lakshmi (whose pregnancy Singam gambled with for vote) and his area people, but not before passing out due to fatal injuries sustained.In the hospital, Singam realizes his folly for betraying trust of his people against their love they showered over him over the years and inspired by Vasu and the final video recording of his alter ego in which he reveals the hideout of remaining money through Vasu, he reconciles with Manimegalai after thrashing her pervert boss who sexually abused many women thereby shaving is head in public and slowly regains his reputation to emerge as the people's favorite MLA candidate in upcoming state elections by performing his duties with the money he stoved away. Fearing his defeat certain and fumed with rage about loss, Rayappan kidnaps J.K Nagar's 40 children to extract the remaining 40 crores from Singam to avenge his loss only to be exposed to media about his atrocities to public and CM, who orders Rayappan to drop his plans not before sacking him from party and issuing orders to arrest Damayanti. In the meeting with CM Nagarajan Chozlan aka Ammavasa, Singam quickly cements his place as CM's right-hand man and becomes J.K Nagar's MLA by playing in the same manner he did with Rayappan, much to the shock and belief of Vasu, who again questions his personality, to which Singam replies there is always a need for a perfect mix of good and evil to sustain in politics.