Jenny Nayar (Goswami), an employee of UK based Barron Industries, introduces a new technology in which objects from the digital world can be brought into the real world, using the wireless transmissions from multiple wireless devices. Shekhar Subramanium (Khan), an employee of the company, faces his last chance to come up with a video game with a difference. In order to impress his son Prateek (Verma), and upon the request of his wife Sonia (Kapoor), he takes his son?s idea that the antagonist should be more powerful than the protagonist.
Akashi (Wu) provides the moves, Jenny does the programming and Shekhar gives his face to the protagonist G.One, whilst the antagonist Ra.One is faceless with powers substantially higher than those granted to G.One. The game has three levels, and either of the players can only be killed in the third level with a special gun that has only one bullet. While designing the game, Akashi notices some malfunctions, but ignores them. When the game is finally launched, it receives a standing ovation and Prateek loves it so much that he insists on playing it instantly. He logs in under the alias 'Lucifer' and goes to the second level, but is interrupted by Akashi as it is time to go home. Ra.One, being unable to end his turn with Lucifer, says that Lucifer shall die.
After the mainframe doesn't shut down, Akashi calls Shekhar who notices something wrong. Ra.One uses the initially introduced technology to come to life, kills Akashi, and goes to find Lucifer. Shekhar rushes home but is blocked by Ra.One on the way. In an attempt to save his son, Shekhar claims that he is Lucifer. However, Ra.One scans his I.D., and kills him for faking it. Upon noticing the strange case of his father?s death, Prateek realizes that Ra.One has come to life. Along with Jenny, he goes to Shekhar's workspace and tries to bring G.One to life. Sonia arrives and asks Prateek to leave with her for India. Ra.One, having taken the form of Akashi, chases them, but G.One enters the real world through Jenny's computer and saves them by initiating a gas explosion which destroys Ra.One into constituent cubes levelled along the road, and takes his H.A.R.T. Finding it impossible to leave him, Sonia takes G.One to India with her through Shekhar?s passport. G.One promises her that he will protect them from any harm. Meanwhile, Ra.One returns to life and takes his humanized form of a model on a billboard (Rampal) and goes after G.One.
During Prateek's birthday party, Ra.One hypnotizes Sonia, assumes her form and kidnaps Prateek. He instructs G.One to give him his H.A.R.T. back and sends the real Sonia in an uncontrollable local train in Mumbai. G.One saves Sonia just at the nick of the time, and then goes back to save Prateek. The game resumes with Prateek controlling G.One's moves. Following a lengthy fight, both of the characters finally reach the third level. With little powers left, the pair trick Ra.One into shooting G.One without his H.A.R.T. attached to him. Furious, Ra.One creates ten alternatives. Prateek, unable to differentiate the real one, asks G.One to quote one of Shekhar?s sayings stored in his folder: ("If you join the forces of evil, its shadows shall always follow you"). The pair then realize that only one of the ten Ra.Ones has a shadow. G.One shoots him and transports himself back to the digital world after absorbing Ra.One's remains. Several months later, Prateek and Sonia return back to the UK, where the former finally manages to restore G.One back to the real world.
source : wikipedia