In 1989, Jayalalithaa (nicknamed Jaya or Ammu), a member of the Tamil Nadu Legislative Assembly and questions the current chief minister, M. Karunanidhi, about the corruption charges, arrests and eavesdropping leveled against her and her party. She demands his resignation and upholds the preceding CM, M. G. Ramachandran, as someone who truly served the people. However, the opposition questions her relationship with MGR, resulting in a brawl after Jaya is slapped. Some ruling MPs attempt to disrobe Jaya; comparing herself to Draupadi, Jaya pledges she would return to the session only after becoming CM.
In 1965, MGR is at the peak of his film career; producer R M Veerappan fires Saroja Devi and even burns all traces of her character in his next film after she shows affection to MGR as Veerappan believes that MGR must stay celibate to preserve popular perception of him as a "god". Jaya, then 16 years old, is pushed by her mother Sandhya, a yesteryear actress, to act. Jaya replaces Savitri, but her unenthusiastic attitudes manifests itself in haughtiness. After realizing her mother just want her to succeed in career and after she is touched by MGR's respect to a crew member hurt on set, Jaya takes a softer stand. The film is a success, so MGR and Jaya shoot more films together and grow fonder. Veerappan again tries to ward off Jaya but she uses her wit to make him back off. Still, Jaya is hurt knowing that she's the other woman in eyes of public, as MGR is married to Janaki. Jaya repeatedly says she touches her hero thinking as if she hugs her mother, she considers MGR as protector like her father, her brother. She realizes she is not respected by few people close to MGR, when MGR was shot in the neck and hospitalized. As MGR recovers, the coalition of party wins. Veerappan hires another heroine for MGRs next film and offers full and final settlement to Jaya to stop her continuous lead pairing with MGR. Jaya tries to discuss with MGR but he brushes of saying it is matter of one film. To retaliate, Jaya praises MGR's rival Sivaji Ganesan in a press conference and makes a hit film with him while MGR's next film becomes flop. MGR apologizes and gets her to star in more films with him.
MGR enters politics with his friend Karunanidhi, but his film schedule interferes with his party duties, leading to his ouster. MGR meets Jaya to tell her he has chosen politics over career in films and leaves with Veerappan in 1973. Meanwhile, her mother Sandhya's demise affects Jaya to a great extent. On board a flight, she is seated beside MGR only to know why he couldn't attend her mother's funeral. Jaya says she cannot be the other woman in his life anymore and does not want to meet secretly to have conversation. MGR wins the election to be the Chief Minister again while Jaya is offered film for playing the character of an elder sister in 1981. However, Jaya she refuses such offers. Jaya accepts a stage performance at a government event in Madurai, where MGR is the guest. He meets her backstage to discuss their memories. By whistling, he takes a moment to appreciate her performance in privacy and wants her to join politics. However, she refuses initially because politics is something she dislikes.
Touched by the condition of girls and other children's education in schools, Jaya highlights the midday meal services under MGR's name by serving him the unhygienic food at his office. MGR inspects the mid-day meal service himself. He advises Jaya to take up the mid-day meal service program for the state to which she agrees and forays into politics. She is fondly called â€œAmmaâ€ by locals. She markets the food scheme door to door and continues to partner with MGR, politically. Jaya takes the portfolio of education, also. Meanwhile, elections are around the corner and a saint is killed. The opposition takes advantage of the situation and says the current political party has â€œno man leftâ€ as they are under Jaya's influence in the party. In a political address to clear the air, stones are pelt at her. To stop the pelting, she speak in opposition of MGR. However, she successfully changes the opinion of the crowd, pointing out and is assigned the job of the party's propaganda secretary, and ends up running in winning that district. Now an MP she is sent to Delhi to meet the prime minister, Indira Gandhi, but is blatantly barred from doing so.
In a Rajya Sabha session, Advani requests the speaker let Jaya speak about South India's situation. Boldly speaking on the disregard of South India and the issue of splitting power generated from the Kalpakkam electricity project, an impressed Gandhi allows her in. Jaya creates a controversial alliance with Congress that was heard by MGR through news. Seen as a betrayal, Jaya was expelled from the position of secretary by Veerappan while MGR is flown to the US for treatment, leading to a dire situation. However, Rajiv Gandhi calls up Veerappan to let Jaya campaign for the party during MGR's absence. The campaigning saves the party and it continues to stay in force. On his return from USA, MGR and Rajiv Gandhi discuss on how Jaya kept the spirit alive. Impressed again, MGR plans to meet Jaya on a dinner, prepared at her home in 1987. Jaya awaits MGR overnight only to know in the wee hours that he is no more. Post MGR's demise, Veerappan wants MGR's wife Janaki Ramachandran to take the responsibility of MGRs position but Janaki accepts her defeat and steps back from politics, saying Jaya has done more for the public. Thus, Karunanidhi takes the reigns of power.
Back in 1990, a lorry runs into Jaya's car as part of a plot against her. Despite her situation, she manages to campaign and her party wins nearly every seat. Finally, she enters the legislature again as CM and gets the respect she deserves.