Plot

A video game company inadvertently creates and lets loose into the real world a single minded villain obsessed with defeating his archnemesis, a young child, in this fast paced sci-fi action thriller.

Jenny Nayar (Shahana Goswami), an employee of the UK-based company Barron Industries, introduces a new technology at a conference. The technology allows objects from the digital world to enter the real world through wireless transmissions. The company is owned by Mr Barron (Dalip Tahil).

Shekhar Subramanium (Shahrukh Khan), a game designer who also works for the company and has delivered a spate of commercial failures, is given a final chance to devise a video game which will actually succeed. Barron says that if the team fails again, he will have to shut the department down. Shekhar's new game is aimed at kids.

Shekhar's son Prateek is an avid gamer, but he is easily bored as most games follow a predictable pattern in which the hero is more powerful than the villain and is able to easily kill the villain in the end. Prateek plays under the gaming handle "Lucifer". Prateek does not consider Shekhar to be a hero or a role model of any kind as Shekhar is very meek and docile in his nature. Shekhar is a South Indian, and very traditional in his views. He is not good at sports and is not "cool". All of this makes Prateek feel that he is not a good dad.

In order to impress his skeptical son Prateek (Armaan Verma), and upon the request of his wife Sonia (Kareena Kapoor), Subramanium uses his son's idea that the antagonist should be more powerful than the protagonist.

Sonia is doing a thesis on Hindi abuses and plans to publish her work in India, for the benefit of all women. Sonia is disgusted that all abuses in Hindi are women based, and she plans to change that and bring more abuses which are male based.

Shekhar's work colleague Akashi (Tom Wu) provides the movements of the game characters, while Jenny designs the game's program. She uses Shekhar's face as a model for that of the game's protagonist (G.One), while the antagonist (Ra.One, Random Access One) is faceless and has substantially greater powers than those granted to G.One, including shape-shifting and mind reading. Ra.One is imbibed with a self-learning AI. Without the team knowing, Ra.One becomes self aware, and starts programming new features for itself that were not provided by Jenny.

Each player possesses a H.A.R.T, (Hertz Amplifying Resonance Transmitter) a special device which gives the characters their powers. The powers of the HART increase with every level. The game, titled "Ra.One," has three levels. Either of the players can be killed only in the third level with a special anti-HART gun that holds only one bullet, with the condition that the player has his H.A.R.T attached at the time of the attack. When tests are conducted, Akashi notices peculiarities in the game but ignores them. Akashi notices that during the game Ra.One is able to pass his movements into the physical world through its bio-mechanical controllers. Akashi also notes that the blows landed by Ra.One on the G.One player can be felt by the player through their G.One bio-mechanical suit.

When the game is finally launched, it receives a standing ovation and Prateek is so enamored that he insists on playing it immediately. The player playing G.One has to don a special bio-mechanical suit to enter the game and fight Ra.One. Prateek logs in under the alias "Lucifer" and proceeds to the second level but is interrupted by Akashi. Ra.One, unable to end his turn with Lucifer and angry that a player has proceeded so far, becomes determined that Lucifer will die.

Ra.One uses the technology introduced at the beginning of the film to enter the real world, a process which causes the mainframe to malfunction. Akashi informs Shekhar of the problem; the latter partially understands the situation. Shekhar finds his lab destroyed and realizes that Ra.One has escaped the game to enter the real world. He had seen messages on screen that said that Ra.One's mission was to kill Lucifer.

Meanwhile, Ra.One kills Akashi and assumes his form. Shekhar sees Akashi being thrown out of the mainframe room. Fearful of his son's life, he rushes home. Ra.One then goes to find Lucifer, and meets Shekhar in the process. In an attempt to save his son, Shekhar claims that he is Lucifer. However, Ra.One scans Shekhar's I.D. and kills him for lying.

After Shekhar's funeral, Sonia tells Prateek that the family will return to India. Later, Prateek notices digital patterns (in the game Prateek had noticed that every time Ra.One fired a blast, it left a spiral pattern on the surface where the blast impacted. Prateek found the same spiral pattern on the road, where Shekhar was found dead) left on the scene of his father's death, and realizes that Ra.One has come to life. He and Jenny attempt to bring G.One to life in a manner similar to Ra.One. Ra.One chases the family on their way to the airport, but G.One enters the real world through Jenny's computer and causes a gas explosion which temporarily destroys Ra.One.

G.One takes Ra.One's H.A.R.T. and accompanies the family to India, after Sonia realizes that she cannot leave him. G.One promises to Sonia that he will protect Prateek from harm. Subsequently, Ra.One returns to life, takes the form of a billboard model (Arjun Rampal), and goes after G.One and Prateek.

At Prateek's birthday party, Ra.One hypnotizes Sonia, assumes her form and kidnaps Prateek. Ra.One then instructs G.One to give him his H.A.R.T. back, and sends the real Sonia in a Mumbai Suburban Railway train. The train crashes into the Chatrapati Shivaji Terminus and destroys it, but G.One saves Sonia just before the crash. He returns to save Prateek, and consequently resumes the game with Prateek controlling him. After a series of fights, both the characters reach the third level.

With little power left, G.One and Prateek trick Ra.One into shooting G.One without his H.A.R.T. attached. Since only one bullet was available, Ra.One is left helpless; furious, he creates ten copies of himself. Prateek, unable to differentiate the real Ra.One, asks G.One to quote Shekhar: ("If you join the forces of evil, its shadows shall always follow you").

The pair realize that only one of the ten Ra.Ones has a shadow: the original one. G.One shoots and destroys him, absorbs Ra.One's remains and finally disappears. Several months later, Prateek and Sonia return to London, where the former finally manages to restore G.One to the real world.