When Bani Chowdhury (Waheeda Rehman) leaves her cousin Maya's (Ashita Mazumdar) house and goes to live with her kindly uncle, a retired army captain (Hari Shivdasani), new avenue of happiness is opened to her. For the first time in her life, she finds herself completely free to do anything, to go anywhere, whereas in her cousin's house, she always had to do what Maya and her husband (Rehman) wanted her to do. She has lived with them on a sort of family charity, for Maya's father who was also Bani's paternal uncle, has provided in his will that his daughter Maya should support her cousin Bani until the latter marries. According to his will, Bani is also to receive Rs. 25,000/- when she marries. The rest of the fortune has gone to Maya because she is the only daughter.
Bani has never bothered about the money that is to come to her on marriage and when she leaves Maya's house, her only thought is to be completely independent. Of course, she can't be a burden on her bachelor, military uncle, however-much he may like to have her live in his house. So, Bani gets herself a stenographer's job in a solicitor's office. Fortunately, the solicitor Ajoy Kumar (Guru Dutt) turns out to be quite young, handsome and susceptible. Soon Ajoy and Bani find them selves in love and make plans for the ultimate realisation of their culminating happiness. But at this time, a tremendous tragedy steals into their life.
Bani's cousin, Maya, is found dead in first class compartment at a railway terminus in Bombay while returning from Delhi with her new born babe. Bani who has gone to the station to meet Maya, is charged with the crime.
Ajoy is distraught with grief. He is certain that his Bani is innocent. He does not take things lying down and it is here that Ajoy's abilities as an ace private investigator are put to real test. He begins to comb the country for every available clue. He is aided in his search by his virile assistant Motilal Sharma (Johnny Walker) who considers no task too mean to help his employer, making love to servant girls to extract information, trailing dark, sinister characters, setting traps for suspects!
And all the while, Ajoy probes and sifts every little bit of evidence of the crime he can discover and just when everything seems lost and the noose fastened on Bani's neck. Ajoy gets a break. And from then on things begin to happen. From then on, thrills galore, music, comedy: laughter , suspense-in fact all the ingredients of human drama , begin to play their respective parts to wind up our story ultimately on a crashing finale in which evil-doers get their desserts and the happy lovers are reunited . . . . . . !