Bhola (Rajesh Khanna) is an innocent band musician who lives with his physically challenged sister Belu in a village. He needs more money for his sister's marriage and he sets off to Bombay to earn. To make his sister not feel sad for his departure, he sings a song on the way towards the railway station. On the other hand, Bombay city Police department is shocked by a series of diamond thefts which leave no clue. But Inspector Pradhan(Vinod Khanna) suspects the thief to be Ranjith (Rajesh Khanna), who is actually a wealthy diamond businessman, on the account that whenever a theft occurs, he is present there. But he has no evidence and searches for that. He creates a plan with Rita (Mumtaz) to attract Ranjith, and to know his secret plans. Bhola arrives the city and he is called Ranjith in a party. Ranjith who arrives the party is surprised to see Bhola, as he looks identical as him. He immediately conceives a plan. He takes Bhola to his place and reveals himself. He convinces Bhola to act like Ranjith in front of the society as he is suffering from cancer and requires treatment. Until his return from treatment, Bhola has to make believe with everybody that he is Ranjith. Actually, Ranjith makes him as himself so that he can continue with his diamond smuggling, meanwhile, there will not be any evidence, as Bhola is going to be Ranjith everywhere. But he did not reveal the reason to him. He also promises that he would give money for his sister's marriage. Innocent Bhola believes him and agrees to the plan.

Ranjith's girlfriend Ruby trains Bhola to be like Ranjith and he acts like him. Bhola finally learns every mannerism of Ranjith and at an instance, he behaves like Ranjith to Ranjith. He acts as Ranjith in the city and the real Ranjith continues his underground work. And Inspector Pradhan cannot come to a conclusion. Rita moves intimately with Bhola thinking of him as Ranjith, but Bhola falls in love with her. In the village, due to heavy floods, Belu loses everything and comes in search of her brother to Bombay with her dog Mothy. Bhola watches a marriage ceremony on the road and he imagines the bride to be his sister and sings the same song which he sang in the village. Belu, who hears that, runs after him, but Bhola has already left the place. Pradhan meets Belu and helps her to reach the place. Belu is misguided by some men regarding the whereabouts of her brother and try to exploit her, but Pradhan saves her from them and takes her to his home. Ruby, who follows Belu to Pradhan's house informs Ranjith about her. Ranjith, posing like her brother, goes to Pradhan's home and takes her with him.

Bhola finds Ranjith is actually a thief and plans a grand diamond loot. Bhola resists the plan, but Ranjith blackmails him with his sister. Unwillingly, he accepts the plan. Ranjith steals a huge amount of diamonds, but Bhola replaces him by attacking him and leaves the place. One of the stolen diamond pieces has a transmitter and police follow the jewels with the help of it. Belu is confused as to who is her brother among them. After several fights, both Bhola and Ranjith are arrested. Both of them claim themselves as Bhola and confuse everyone. Belu suggests that her brother sing a song which cannot be sung by anyone else. But both sing the song. Finally Bhola and Belu's dog Mothy identifies the real Bhola and Ranjith is arrested and sent to prison. Belu finally marries Inspector Pradhan and Bhola marries Rita.

The music, composed by Kalyanji-Anandji, with lyrics by Indivar, Gulshan Bawra and Qamar Jalabadi, is a strong point of the film.

It was remade in Tamil as Ninaithadhai Mudippavan (Finishes everything he thinks), with M. G. Ramachandran reprising Rajesh Khanna's roles and M. N. Nambiar reprising Vinod Khanna's role, which was released in 1975.

The Tamil Film had some few scenes different from original movie like:

• Ranjith in Hindi kills the doctor who has treated his gunshot wounds, whereas Ranjith in Tamil lets the doctor leave his place alive. Ranjith is portrayed only as a thief and not a murderer.
• In the final court scene, Ranjith is identified by Bhola's dog, whereas in Tamil version Ranjith convinces everybody that the dog is lying. He is finally trapped on seeing his mother lying dead and he reveals himself by exposing his love for his mother.
• Tamil version has a story as to why Ranjith has become a diamond thief, with a small flashback in his childhood, which is not in Hindi movie.

The film was remade in Kannada as Mutthanna starring Shiva Rajkumar.

• 1970 Filmfare Best Actor Award for Rajesh Khanna.