'Gunaah' revolves around an hones lady police officer, Prabha (Bipasha Basu). A lonely woman, she takes upon herself of cleansing the corrupt system by being a part of it. Her life takes a dramatic turn when she meets Aditya (Dino Morea).
Prabha falls in love with Aditya when, while chasing him, she is about to fall off a rooftop. Without a thought, Aditya saves her life even though he knows that he would be arrested in the process. This incident leaves a lasting impact on her mind and more so when Aditya refuses to explain his actions in the lock-up as well as in the court.
When Prabha takes Aditya down memory lane, she discovers that he is a nice person, who was forced to take the path of crime by a wrongdoing of the system. The two feel drawn to each other and fall in love.
Prabha starts making an effort to know the reasons that made Aditya choose the path of crime and reform him in the process.