Danish (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 grew up, Danish 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, Danish leaves for Mumbai.

Danish gets into Film City past its guards and finds shelter in an actor's vanity van, hiding under the sofa. Traveling around in the empty van, Danish 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 her director, Anay Verma (Ivan Rodrigues), but he rejects casting a mute hero. Akshara's father, a doctor, treats Danish after he gets into a fight with security guards. On his recommendation, Danish and Akshara arrive at Finland to meet his friend who shows Danish their new "Live Voice Transfer Technology", that uses embedded micro-recorders and ear-pieces and enables mute people to communicate via a "borrowed" voice.

Akshara and Danish return to Mumbai with the equipment and find Amitabh Sinha (Amitabh Bachchan), a failed actor and drunkard. Although he was rejected because of his deep baritone, the duo is actually impressed with the same baritone and ask him to lend his voice to Danish; he accepts and signs an agreement not to disclose himself as Danish's voice. Their first project becomes a major success and numerous film offers come for Danish, now named "Shamitabh" (a portmanteau of Danish and Amitabh).

Amitabh, who is officially Danish's valet, also wants a role in accepting or rejecting scripts. He tells Danish his voice is as big — if not bigger — a star than Danish's talent. The friction between them grows. Amitabh gets fever and Danish makes him take an injection. In retaliation, Amitabh makes Danish accepts singing a song in his new film. Amitabh sings the song "Piddly", but is horrified by its picturization of toilets across snowy peaks, suggested by Danish to tease him.

Danish arrives in London to deliver a speech which Amitabh voices for him, but Danish treats him like a servant. Amitabh later becomes inebriated and attacks a police officer, leading to his arrest, but Danish bails him out.

When the duo returns to Mumbai, journalists ask Danish if he is apologetic about the incident. Amitabh mischievously makes him reply in negative, much to Danish's fury. The duo parts ways; Danish 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 Danish, returns but berates Danish when he refuses to apologize to Amitabh. She forces Amitabh and Danish to reconcile.

A media reporter, having discovered the secret that Danish was born mute, threatens to expose the duo but they decide to tell the truth and surprise the world. Danish and Amitabh accept to work in Akshara's debut film, and Danish tells Akshara that he loves her. While Danish and Amitabh are on the way to Akshara's film ceremony, a car accident occurs; Danish is killed while Amitabh's larynx is damaged, making him mute.

Sometime later, Amitabh paces around Danish's grave with Akshara's script, imagining that Danish is still alive and practicing his lines for the film.