Jenny Nayar, portrayed by Shahana Goswami, is an employee at Barron Industries, a UK-based technology firm. She presents an innovative technology at a conference that merges digital and real worlds by allowing objects to enter reality through wireless transmission. Meanwhile, Shekhar Subramanium, played by Shahrukh Khan, is a game designer facing a last shot at redemption after a series of flops. To win the approval of his critical son, Prateek, played by Armaan Verma, and to honor his wife, Sonia, portrayed by Kareena Kapoor, Shekhar decides to use Prateek’s idea: the antagonist must be stronger than the hero.
In collaboration with his colleague Akashi, played by Tom Wu, who animates the game characters, and Jenny, who creates the game’s coding, Shekhar channels his own features into the design of the protagonist, G.One, while the antagonist, Ra.One, remains faceless. Each player engages with a special device known as a H.A.R.T, endowing the characters with extraordinary abilities. The game, aptly named "Ra.One," consists of three intense levels, with a unique twist; players can only be eliminated in the final level with a single bullet gun, provided they have their H.A.R.T connected. Despite some abnormalities noted by Akashi during testing, he brushes them aside.
Upon its release, the game garners an enthusiastic response, and young Prateek eagerly logs on under the alias "Lucifer" to experience it. However, as he journey through the stages, he unknowingly draws the ire of Ra.One, who seeks vengeance for the disruption of his turn. Harnessing the technology from earlier in the film, Ra.One breaches into reality, leading to a malfunction in the mainframe system. When Akashi alerts Shekhar about the unfolding disaster, his concern escalates, prompting him to rush home to safeguard his son.
Tragically, in the ensuing chaos, Ra.One kills Akashi and adopts his appearance to confront Shekhar. Desperate to protect Prateek, Shekhar claims to be Lucifer, yet Ra.One scans his identity and ruthlessly kills him. Following Shekhar's funeral, Sonia tells Prateek it is time to return to India. However, with remnants of his father’s digital imprint lingering, Prateek realizes Ra.One is still a threat. Teaming up with Jenny once more, they endeavor to awaken G.One, mirroring Ra.One's emergence into reality.
As Ra.One pursues the family to the airport, G.One materializes through Jenny's computer, leading to a gas explosion that disorients Ra.One momentarily. Acquiring Ra.One's H.A.R.T during the chaos, G.One travels with them to India, pledging to protect Prateek after Sonia decides she cannot abandon him. However, the menace returns in a new guise, transforming into a billboard model played by Arjun Rampal, and sets his sights on punishing Prateek and G.One.
During Prateek's birthday celebration, Ra.One hypnotizes Sonia, kidnaps Prateek, and demands G.One return his H.A.R.T. The real Sonia is dispatched on a suburban train headed for disaster, but G.One heroically saves her just before the crash. Back to the confrontation, G.One and Prateek navigate through intense battles, finally reaching the game’s pivotal third level.
In a climactic showdown with dwindling strength, G.One and Prateek manage to deceive Ra.One into shooting G.One without his H.A.R.T, leaving him vulnerable. In a fit of rage, Ra.One replicates himself into ten terrifying copies. To uncover the true Ra.One, Prateek invokes a line from his father: > “If you join the forces of evil, its shadows shall always follow you.” This prompts G.One to identify the original Ra.One by recognizing that only he casts a shadow. With a decisive shot, G.One dismantles him, absorbing his fragments before fading away.
Months later, Prateek and Sonia find themselves back in London, where the former successfully restores G.One to existence once again in the real world.
