Bhola, Rajesh Khanna, is an innocent band musician who lives with his physically challenged sister Belu, Kumari Naaz, in a small village. He needs more money to arrange Belu’s marriage and sets off for Bombay to earn it. To keep Belu from feeling sad about his departure, he softly sings a farewell song on the road to the railway station. In Bombay, the city’s Police Department is stumped by a string of diamond thefts that leave no clues behind. Inspector Pradhan, Vinod Khanna, suspects the thief to be Ranjeet Kumar, a wealthy diamond businessman who seems to appear at every crime, but there is no concrete evidence to prove it. To uncover the truth, he teams up with Leena, Mumtaz Askari, hoping to lure the culprit and expose his plans.
Bhola arrives in the city and attends a party where he is mistaken for Ranjeet Kumar. The real Ranjeet, arriving later, is startled by their identical looks and quickly hatches a plan of his own. He reveals himself to Bhola and convinces him to impersonate Ranjeet in public while he goes away for cancer treatment, insisting that Bhola’s act will shield him from suspicion. He also promises money to help Belu’s marriage. Trusting the arrangement, Bhola agrees to play the part.
Ranjeet’s girlfriend Ruby, Faryal, trains Bhola to mimic every gesture and mannerism of the crime lord. Bhola masters the routine so well that he can even fool the real Ranjeet when they cross paths. Leena grows intimate with Bhola, thinking he is Ranjeet, while Bhola himself begins to fall for her too.
Back in the village, heavy floods devastate Belu’s life, and she travels to Bombay with her dog Moti, Rexy. Bhola spots a wedding procession and hums the same song he sang back home, which Belu hears and tries to reach, but Bhola has already moved on. Inspector Pradhan helps Belu reach Bhola’s quarters, though some men mislead her about his whereabouts and try to exploit her. Pradhan protects Belu and brings her to his home, while Ruby follows Belu to Pradhan’s house and informs Ranjeet, who arrives posing as Bhola’s brother to take Belu away.
Bhola learns that Ranjeet plans a grand diamond heist and fears for Belu’s safety. Under pressure, Bhola reluctantly agrees to participate. Ranjeet steals a huge stash of diamonds, but Bhola counters him by turning the situation to his advantage and escaping with the jewels. One of the stolen diamonds carries a transmitter, and the police track the haul with its help. Belu, meanwhile, remains unsure about which man is truly her brother.
After a series of confrontations, both Bhola and Ranjeet are arrested and claim to be Bhola, creating confusion. Belu asks for proof, and with the help of her dog Moti, identifies the real Bhola. Ranjeet is arrested and sent to prison. In the end, Belu weds Inspector Pradhan and Bhola marries Leena.
