Auro (Amitabh Bachchan) is an intelligent and witty 12-year-old boy with an extremely rare genetic disorder called progeria. Mentally he is twelve and very normal, but physically he looks five times older. In spite of his condition, Auro is a very happy boy. He lives with his mother Vidya (Vidya Balan) who is a gynaecologist.

Amol Arte (Abhishek Bachchan) is a young, cold blooded politician. He is out to prove to the world that 'politics' is not a bad word. He is a man with a mission. Auro is Amol's son; however, Vidya conceals this from him. Amol meets Auro when he visits the boy's school. Though he does not know that Auro is his son, he takes him to Delhi to see the president's house. Auro, who knows the truth, tries to get his mother and father back together, but Vidya resists, still hurt by the fact that Amol wanted her to have an abortion when they first found out she was pregnant.

Amol realizes his mistake though, and proposes to Vidya, as he is still in love with her. He stays by Auro's side when he finds out that Auro is his son. Auro's health begins to deteriorate as he reaches his 13th birthday, his physical defects catching up. However, he is finally able to reunite his mother and father once again as Vidya gives in to her feelings for Amol and her motherly love for Auro.

Paa is a rare story about a father-son, son-father relationship.