With many twists and turns, this lost-and-found family drama revolves around Govind (Amitabh Bachchan) and Rajesh (Vinod Mehra), two brothers who are happy being brought up by their considerably older stepbrother Hari (Sanjeev Kumar). However, when Hari has to leave home for two months to complete his law degree, his newly married wife Seema (Tanuja), jealous of her husband's excessive affection for the two, misbehaves with them, forcing them to leave home and escape to Mumbai.
After days spent doing menial labour, the two brothers are finally given a home by the widower Rahim, who has a son Anwar and daughter Farida. Govind takes on the responsibility of providing for his brother's education by becoming the taxi-driver Chhotu Ustad. Rajesh, however, is ambitious and foppish, and spurns his brother and the latter's pride by marrying the rich Seth Verma's daughter Manju (Bindiya Goswami) and staying at his father-in-law's mansion.
Verma's brother Bansi (Prem Chopra), who has actually been instrumental in fixing up this marriage, employs the unsuspecting Rajesh to carry on a drug-smuggling trade using the vehicles of he Verma Transport Company, owned by the Seth. One such operation is foiled by the police, and Anwar, also mixed up in his business, is left seriously injured. Bansi's men attempt to kidnap Anwar to destroy evidence when Govind is not at home. In the skirmish that ensues, Rahim is shot fatally.
In the aftermath of the tragedy and Rajesh's realisation of how he has been used by Bansi, Rajesh seeks his brother's mercy. However, at this very moment, Bansi murders Seth Verma and tries to frame Rajesh for it. However, it is Govind who is found at the scene of the crime and accused of murder. The judge at his trial is Hari, who is now one of the leading legal eagles in the city. In the course of the trial, the real identity of the accused and his brother are revealed to Hari. Hari now steps down as judge and defends Govind. Through a series of astute investigations, he unearths the truth. Bansi, cornered, attempts to shoot Hari and escapes the courtroom, but is finally tracked down by Govind in his beloved taxi 'Basanti'. As the smuggler is arrested, the three brothers are reconciled.