Don movie is based on the Don (Amitabh Bachchan) who is the leader of an underworld gang until Police Officer D'Silva (Iftekhar) tracks him down and in an attempt to catch him ends up killing him. But Don's death is only known to D'Silva. So Officer D'Silva tracks down a lookalike of Don named Vijay. Vijay impersonates Don to take down the rest of Don's gang while the DSP promises him that he will give a proper education to his wards who are actually the children of Jasjit (Pran).
Meanwhile, Jasjit comes out of prison after serving his term, only to find Officer D'Silva having a red diary that would show Vijay's true identity. Officer D'Silva is killed by Malik who is introduced as the Interpol officer, but is in fact the real king of the underworld. It's now a race to retrieve the diary in which the DSP has documented the substitution of the real Don with the innocent Vijay.