A romantic comedy, co-starring Amitabh Bachchan, Sunil Shetty and Dia Mirza, Kyon Ho Gaya Na tells the story of a sensitive, sensible girl Diya Malhotra (Ash) and an adventurous, reckless guy named Arjun Khanna (Vivek).
Diya has been brought up by her father (Tinu Anand) after her mother passed away while she was just a child. She spent her childhood in the picturesque coffee-plantation town of Coorg. As a young woman she becomes interested in social work and starts doing her Masters degree in the respective field.
Diya has been involved in a little bit of social work at the place of her father?s friend Raj Chauhan (Amitabh Bachchan), a confirmed bachelor who has dedicated his life to a small orphanage in his home.
She goes to Mumbai for her exams for Masters degree. There, she meets an adorable Arjun Khanna (Vivek) in quite unusual circumstances. By nature Arjun is an easy-going fellow with a knack for adventures and motorcar racing.
To Diya?s surprise it turns out that she will be staying in Mumbai with Arjun?s family (Om Puri and Rati Agnihotri) since they are friends of her father.
Living together under the same roof, Diya and Arjun get to know each other better. They develop care and affection for each other but they are also often at loggerheads due to their differing opinions on love and marriage. Little do they know that these very differences will one day form an unbreakable bond between them?