During one of the fights, she chases a boy and accidently lands to a boxing gym. She is intrigued by knowing that the coach Narjit Singh, who also happens to the coach of the Asian Champion Tinko Singh. Kom ask him about her boxing aspirations. Later Coach Singh tells her to visit the gym for next 30 days and will only teach her if she is deserving. She starts visiting the gym without telling her father, though her mother is informed about her joining the gym. Day passes and Kom is not looked by him. However, due to Kom's dedication and stubbornness Coach Singh starts training her. During this time Coach Singh changes her name from Mangte Chungneijang Kom to M.C. Mary Kom. Kom wins the state level Championship and her father confronts her for hiding and playing boxing. Her father asks her to choose between him and Boxing; with Kom choosing boxing. Later, Kom had to fight a wrestler in order to arrange money to get her household cow back, where she meets the footballer Onler Kom. Mary and Onler starts liking each other. Kom's father, who had stopped talking since the confrontation, supports her after during watching the 2002 Women's World Amateur Boxing Championships on television. Kom went on to win the championship and her father apologise to her for not understanding her passion for the sport. Onler Kom proposes to Mary Kom as he always liked her and he would never ask Mary to leave boxing. After winning the 2006 Women's World Amateur Boxing Championships, Kom agrees to marry Onler; however her Coach shows his unhappiness towards Kom's marriage decision.
After her marriage, Kom becomes pregnant and started missing boxing. Later, she realises to give up her career for her children. Then, the film shifts to current time showing Kom's delivery in hospital. She gives birth to twins and becomes busy with her children. Kom then applies for a government job; to her surprise she is offered a job as a constable, which she refuses. She is then get unrecognised by people in a bus, which devastates her. She is then encouraged by Onler to train again in boxing. She joins the gym again and her husband looks on the children at home. Her Coach is still angry about her decision of marriage. Kom makes a come back in boxing with National Boxing Champion ship; however despite scoring better than her opponent, she looses the match due to partiality of the judges. Angry and confused, Kom throws a chair towards the judges, causing a banned on her. She later gives a written apology and the officials insults her during accepting it.
Kom then reaches to her Coach asking to train her as he is the one who can get the best out of her. Her Coach starts training her with gruelling and tough activities. Kom reaches the finals of the 2008 AIBA Women's World Amateur Boxing Championships. Onler Kom informs her about one of her son's hole in the heart before the match. A heartbroken and shattered Mary Kom fails to defend her and is constantly hit by her opponent. After a knockout punch, Kom sees her husband and children in audience. She gains strength and fights her opponent winning the 2008 Women's World Amateur Boxing Championships. She is later informed about the successful surgery of her son on the podium, accepting the medal. She is given the nickname "Magnificent Mary" and the film ends with national anthem, showing Indian flag rising above.