Nasir Hussain, as someone (he himself?) once remarked, came to Bombay with one story in his briefcase, and made out of it one blockbuster after another. The story of a son, separated by circumstances from one parent and going through various ups and downs (including falling for the distant parent’s foster offspring, being impersonated by a crook, etc) before the happy ending, was one that was played out in Tumsa Nahin Dekha, Dil Deke Dekho, Phir Wohi Dil Laaya and Pyaar ka Mausam.
But, contrary to popular belief, Nasir Hussain was by no means a one-trick pony. He had other plot elements up his sleeve as well, and they appear now and then sporadically in various films. The ‘couple promised to each other as children’ trope is one [which always ends up with the couple—completely unaware of having been ‘betrothed’ in childhood, even sight unseen—falling in love with each other]. Another was the hero being [mistakenly, of course] believed to have killed a sister [or sister figure] of the heroine’s, after having played fast and loose with her—this, naturally, causing serious heartache and betrayal for the heroine until she realizes that her beloved couldn’t possibly do something so heinous.