Auro (Amitabh Bachchan) is an intelligent, witty 13 yr old boy with an extremely rare genetic defect that causes accelerated ageing. He suffers from a progeria like syndrome. Mentally he is 13, very normal, but physically he looks 5 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 young, progressive and a full of ideals, 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. Paa is a 'rare' story about a father-son, son-father relationship.