It's a story about Zara, a Pakistani Muslim girl having a Sikh nanny (who migrated to Pakistan following her Muslim employers). Before her nanny die, she asked Zara to bring her ashes to a river in Punjab, India.
This Zara went to India, while crossing the border her bus crashed and being saved by Veer, a Hindu Indian soldier. Veer was attracted to Zara and helped her to go to that Sikh holy place.
Later, Veer went to Pakistan to search for Zara, but because Zara was engaged to the son of a powerful Pakistani politician - Veer was caught and jailed - accused of being Indian spy. He was jailed without no trial for decades.
Later, a Pakistani Muslim lady (A lawyer) decided to help Veer and later found out that while Veer was jailed while searching for Zara in Pakistan. Zara went to India searching for Veer, for decades she's been helping Veer's family and even manage an orphanage in India.
Finally, Veer being released and met Zara and live happily ever after.
Veer - Shah Rukh Khan
Zara - Preety Zinta
Pakistani lawyer - Rani Mukherjee
Veer's father - Amitabh Bacchan.