Mammo is about the story of a lady called Mehmooda Begum (Farida Jalal), who is fondly known as 'Mammo'. Mammo was married to a man in Lahore, and like many other Indian Muslims, following the Partition, Mehmooda Begum became a Pakistani citizen. But when her husband died, Mammo was left to uncaring in-laws and soon there were hassles over property. Left to herself and with nowhere to go, Mammo comes to Bombay on a tourist visa and moves in with her sister Fayyazi (Surekha Sikri) and her orphaned grandson.
Threatened with deportation and no longer able to extend her visa, Mammo pays a tout to cross her name off the register of foreign visitors. But this trick backfires and Mammo is put onto a train with other illegal immigrants and returned to Pakistan. Shyam Benegal sensitively portray's the problems and conditions of many Muslim families who were pulled away by the Partition.