Black Mail (1973)

Kailash, a young scientist working on a formula to harness solar energy, falls in love with Asha and wins her through a series of anonymous love letters. Their marriage is happy until an enemy from Asha's past and a gang of criminals hungry for Kailash's invention begin blackmailing her, turning love letters and secrets into weapons. Vijay Anand, one of the great stylists of Hindi cinema, mixes romance and espionage-style suspense. The film's best-remembered element is the Kishore Kumar song "Pal Pal Dil Ke Paas", one of the most popular romantic songs of the 1970s.