A humanoid alien (Aamir Khan) lands on Earth naked on a research mission in Rajasthan but is stranded when the remote control for his spaceship is stolen. He manages to get the thief's cassette recorder, a Panasonic RQ-565D.

On the same day in Bruges, Jaggu meets Sarfaraz and falls in love with him. Jaggu's father objects to their relationship because Sarfaraz is a Pakistani Muslim, while Jaggu is an Indian Hindu. He consults godman Tapasvi Maharaj who predicts that Sarfaraz will betray Jaggu. Determined to prove them wrong, Jaggu proposes to Sarfaraz, who accepts. She is heartbroken at the wedding chapel when she receives a letter calling off the marriage due to their cultural differences.

Jaggu returns to India and becomes a TV reporter. She meets the alien and is intrigued to see him distribute leaflets about the "missing" God. She saves him when he attempts to take money from a collection box, earning his trust.

The alien tells her that he is a research scientist from another planet. His people know nothing about dressing, religion, or verbal communication. They transfer ideas by holding hands. The alien learned to fit in among humans by wearing clothes and using money stolen from "dancing cars".

In a flashback, after being accidentally hit by a truck, the alien is befriended by bandmaster Bhairon Singh, who takes him along with his troop. The alien attempts to learn to communicate by grabbing people's hands but they chase him away, believing him to be a pervert. Bhairon takes him to a brothel, where the alien holds a prostitute's hand for six hours and thus learns the Bhojpuri language.

Bhairon has a conversation with the alien and suggests that the thief may be in Delhi. The alien leaves for Delhi. Due to his strange behavior, people assume he is drunk and call him "PK" (pee-kay, Hindi for "drunk"). People tell him that only "God" can help him find his remote. PK sincerely practices Indian religions attempting to find "God", to no avail. He later discovers that Tapasvi has his remote, who claims it was a gift from God, and refuses to return it. Jaggu promises PK that she will recover his remote and he can go back home.

PK conjectures that Tapasvi and other godmen must be dialing a "wrong number" to communicate with God and are therefore advising the public to engage in meaningless rituals. Jaggu encourages the public to expose fraudulent godmen, by sending their videos to her news channel. This "wrong number" campaign turns into a popular mass-movement, to the dismay of Tapasvi. Meanwhile, Bhairon finds the thief and contacts PK, telling him that he sold his remote control to Tapasvi. PK realises that Tapasvi was a fraud all along and that it wasn't a "wrong number". Bhairon informs him that they will come to Delhi, but both die in a terrorist attack. The attack is later declared by Tapasvi's group to be stated as protecting their Gods.

Tapasvi decides to confront PK on-air. Tapasvi asks PK what the "right number" is. PK says that "God that created us all" is the only concept people should believe in and that the other "duplicate Gods" are created by man. Tapasvi argues, saying that PK is trying to take people away from their Gods and that they will not stand for their Gods being taken away. He claims he has a direct connection to God and refers to his prediction of Sarfaraz's betrayal to try to prove that Muslims are liars. PK disagrees. PK realises that Sarfaraz did not write the letter to Jaggu.

She contacts the Pakistani Embassy in Belgium where Sarfaraz worked part-time. The embassy tells her that Sarfaraz still loves her and transfers the line to him, who is now living in Lahore. Jaggu and Sarfaraz reconnect, and Tapasvi is forced to return PK's remote, while also being exposed to the whole of India that he had planned to break Jaggu and Sarfaraz through his "predictions".

Meanwhile, PK has fallen in love with Jaggu but refrains from telling her because she loves Sarfaraz. Having filled multiple audio tapes with her voice alone, he takes two suitcases full of tapes and extra batteries when leaving for home. After his departure, Jaggu publishes a book about him.

One year later, PK returns to Earth on a new research mission on human nature with several other members of his species.