Daanish (Dhanush), born mute in rural Igatpuri, loves films. He acts out scenes before his class, amazing his peers and teacher, and spends all his time in the local Paradise theatre. He even tries to board a Mumbai bus to go become a film star but is pulled off by his mother's friends. When he grows up, Daanish brings pakodas to the local video-store owner and watches films every day. His mother feigns illness and keeps him from going to Mumbai but when she dies, Daanish leaves for Mumbai.
Daanish gets into Film City past its guards and finds shelter in an actor's vanity van, hiding under the sofa. Travelling around in the empty van, Daanish desperately tries to meet directors but cannot. He meets assistant director Akshara Pandey (Akshara Haasan) who is impressed by his talent. She shows his video, shot on her phone, to a director but he rejects casting a mute hero. Akshara's father, a doctor, treats Daanish after he gets into a fight with security guards. He recommends Daanish to his friend in Finland who shows Daanish new technology that uses embedded micro-recorders and ear-pieces and enables mute people to communicate via a "borrowed" voice.
Akshara and Daanish find Amitabh Sinha (Amitabh Bachchan), once rich and successful, but now a failed actor and drunkard. Although he was rejected because of his deep baritone, the duo are actually impressed with his rejected baritone and ask him to lend his voice to Daanish; he accepts, and signs an agreement not to disclose himself as Daanish's voice. Their first project becomes a major success and numerous film offers come for Daanish, now named "Shamitabh" (a portmanteau of Daanish and Amitabh).
Amitabh, who is officially Daanish's valet, also wants a role in accepting or rejecting scripts. He tells Daanish his voice is as big — if not bigger — a star than Daanish's talent. The friction between them grows. Amitabh gets fever and Daanish makes him take an injection. In retaliation, Amitabh makes Daanish accepts singing a song in his new film. Amitabh sings the song "Piddly", but is horrified by its picturisation of toilets across snowy peaks, suggested by Daanish to tease him.
Daanish is invited to London to deliver a speech which Amitabh voices for him, but Daanish treats him like a clerk. Amitabh later becomes inebriated and attacks a police officer, leading to his arrest, but Daanish bails him out.
When they return to Mumbai, journalists ask Daanish if he is apologetic about the incident. Amitabh mischievously makes him reply in negative, much to Daanish's fury. The duo parts ways; Daanish plays a mute person in his next film, while Amitabh dubs for another actor in a different film. Both films fail.
Meanwhile, Akshara, who had gone to Sikkim to write a script for Daanish, returns but berates Daanish when he refuses to apologise to Amitabh. She forces Amitabh and Daanish to reconcile.
A media reporter, having discovered the secret that Daanish was born mute, threatens to expose the duo but they decide to tell the truth and surprise the world. Daanish and Amitabh accept to work in Akshara's debut film, and Daanish tells Akshara that he loves her. On the way to Akshara's film ceremony, Daanish and Amitabh have a car accident; Daanish is killed while Amitabh's larynx is damaged, making him mute.
Some time later, Amitabh paces around Daanish's grave with Akshara's script, imagining that Daanish is still alive and practising his lines for the film.
