Krishna (Prabhu Deva) is a happy-go-lucky Tamilian (Teluguite in Telugu version) who works in Mumbai. He is originally from Coimbatore (Rajahmundry in Telugu version). He spends his time searching for his dream girl. He wants his life partner to be educated and have supermodel looks. He does not want a country girl for a wife. His grandmother is on her deathbed, and his father (Joy Mathew) wants him to get married. His father, with the aid of his grandmother, chooses a girl from their village. He tries many times to stop the marriage, but nothing works. He marries the girl, Devi (Tamannaah), unwillingly. He tries to get rid of her and shifts to a new house to hide her from his friends.

After moving to the new house, Devi's personality changes, which surprises Krishna. He unwillingly takes her to a film festival where, to his shock, she dances and calls herself Ruby. She attracts the attention of actor Raj Khanna (Sonu Sood). Krishna inquires about the house with his neighbours and, to his disbelief, learns about the past owner, a girl called Ruby, an actress who was supposed to debut with Raj was replaced by another actress, which caused her to fall into depression and commit suicide in that very house. He finds out that his wife is possessed by Ruby. Ruby gets an acting offer for the third sequel of Revolver Raja with Raj, and she forces Krishna to let her attend every rehearsal. Eventually, Krishna makes a deal with her to end this after one movie. Through the film making process which involves Krishna lying to Devi, Krishna begins to appreciate his wife and falls in love with her. Ruby and Krishna also become close friends.

At the release of the movie, Krishna praises Ruby for her acting, but she betrays him and does not leave Devi's body as promised. Krishna tries to make her leave by exposing her in the press conference, but she tricks him. At that point, Raj also proposes to her. She faints and is rushed to the hospital. The doctor reveals that Devi is pregnant with Krishna's child. Krishna begs Ruby to leave his wife's body, and Ruby obeys. The movie ends with Krishna and Devi happy together, while we see Ruby's name on a name tag in a trash bin disappear.

In the Tamil and Telugu versions, after a few years, Krishna and Devi live happily and have a child by the time. Krishna goes out to work, but he is possessed by Ruby, whose name tag is back again. This scene sets the stage for the sequel.

In the Hindi version, Raj is swarmed by his fans asking for an autograph, and in the midst of all, he recognizes a hand stretched out, asking for his autograph. He asked for her name, and she answers, "Ruby" (Esha Gupta). It is understood that Ruby has possessed her body.