I'm not sure specifically about the 'agar tum na hote' drama story as I haven't watched it in detail. But generally, it might be about relationships, love, and the impact of a person's presence or absence in someone's life.
Since I don't know this novel, we can make some educated guesses. Given the title 'zabt e gham', if 'zabt' means something like 'control' or 'capture' and 'gham' means'sorrow' in some languages, it might be about a character's struggle with controlling or coming to terms with their sorrow. It could involve family drama, where the character has to face the pain within the family unit and tries to manage it.