Gumnaam - The Mystery is the story of an aspiring actress Ria (Mahima Chaudhary) who models / acts in music videos but has her eyes set on making it big in films as a lead actress. During one of her shoots, she happens to meet Dev (Dino Morea) a stuntman by profession, who saves her life during an accident on the set. She's impressed with him, soon love blossoms between them and they are on a romantic high. Meanwhile Ria is offered a lead actress' role in a film by casting director Rishi Gandhi (Madan) but the condition is that she has to leave for Shimla the very next day. Dev is very happy about her getting the role in the film and sees her off, wishing all the luck. On reaching Shimla, she is introduced to the film's director Muni Gandhi (Govind Namdeo) who also praises her beauty and appreciates his assistant's perfect choice of casting her as the heroine.

Slowly Ria begins to see some unusual mystery around her. The director asks her to give a screen test but when she asks for a second take she is refused. His assistant Rishi Gandhi delivers the cassette to Remon (Suman Ranganathan) a rich female staring in a huge mansion in Shimla. Who is this woman and why the cassette has been delivered to her? What happens after the screen test? How Ria tackles the situation and what's she doing in Shimla? All this forms the crux of the story of the high voltage drama Gumnaam- The Mystery