This is Shekhar Kapur's first film and is a loosely based on Erich Segal's 'Man, Woman and Child'. D K Malhotra (Naseeruddin Shah) is a successful Delhi architect who lives an idyllic existence with his lovely wife (Shabana Azmi) and two young daughters played by Urmila (who grew up to be a sizzling star of current Bollywood) and Ardhana. The young family have a happy and comfortable existence until it is revealed that Malhotra has an illegitimate son (played by the child star Jugal Hansraj) from a casual relationship that he had early on in his marriage.

Following the death of the boy's mother, Malhotra is forced to accept the child. Masoom explores the trauma his wife and family undergo as they are faced with this changed reality.