The grief that drives the second story
The main engine of the Don films is the lookalike plot: a street performer named Vijay is pressed into impersonating a crime lord so the police can work their way into Don's organisation. But running alongside that operation, almost from the start, is a quieter and more personal line — the story of Jasjit and Roma.
Roma's brother is killed by Don early in the film. She does not accept this as a loss to be mourned and moved on from; she accepts it as a debt to be settled. Her purpose is revenge, and she pursues it independently of the police plan entirely. This matters structurally because it gives the audience a second reason to watch that has nothing to do with procedure or disguise. Where the police plot is cool and tactical, Roma's thread is emotional and unresolved.
In the 1978 original, Roma is played by Zeenat Aman. She enters the story hostile to Vijay — because Vijay, in his role as Don, appears to her to be the man who murdered her brother. The dramatic irony is precise: the person she wants to kill is the one person in the film who is genuinely innocent of the crime. Her anger is entirely justified by the facts as she understands them, and entirely misdirected by the facts as the audience understands them.
Jasjit is the child left behind after the violence — the brother's son, taken in by Vijay after he goes underground as Don. This detail does something specific: it tethers Vijay emotionally to consequences he did not cause, gives him a stake in outcomes beyond his own survival, and provides a domestic register against which the thriller register can play. The child is also a source of information, eventually, about what really happened to Roma's brother.
The revenge thread resolves when Roma learns that Vijay was not Don, and that the real Don was responsible for her brother's death. In the 1978 film this arrives too late — Roma has already spent most of the film turned against the wrong man, while Vijay has survived the criminal operation without the other characters knowing what the audience knows. Roma's revenge becomes retrospectively impossible, which is the point: the film is structured so that grief and justice arrive in the wrong order.
The 2006 remake uses the same emotional architecture but routes it toward a very different destination, one that reframes who Roma is and what she was actually doing throughout. That reversal lands harder precisely because the revenge subplot has been running sincerely for two hours beside it.