PlotA humanoid alien (Aamir Khan), lands on Earth on a research mission in Rajasthan but is stranded when his beacon to his spaceship, a locket around his neck, is stolen. PK, not knowing Earth's customs, learns to fit in among humans by wearing clothes and using money that he steals from couples having sex in 'dancing cars'. He attempts to learn to communicate by grabbing peoples' hands and absorbing their memories through touch, but they chase him away when he tries. The passenger vehicle that carries Bhairon Singh (Sanjay Dutt) strikes him and renders him unconscious. Bhairon takes him to a doctor who declares PK is a person suffering from amnesia. Bhairon assumes the amnesia is as a result of the collision PK has had with his vehicle. Bhairon now knows that the victim of the accident won't be able to complain about the accident. So Bhairon decides to run away. Later, he takes pity, changes his mind and takes PK along with his troop. Over time PK becomes his friend. Later, interpreting the alien's hand-grabbing as sexual interest, takes him to a brothel. There, the alien holds a prostitute's (Reema Debnath) hand for six hours and thus learns the Bhojpuri language.
The alien travels to Delhi in search of his stolen remote. Due to his strange behaviour, the people in the city assume he is tipsy (tipsy translates to pee-kay in Hindi) and call him PK. Delhi turns out to be a very large city, and people tell him that only God can help him find his remote. PK tries to find God, but is confused by India's various religions and their confusing traditions. He later discovers that a godman, Tapasvi Maharaj (Saurabh Shukla), has his remote. However, Tapasvi falsely claims to have attained the object from God in the Himalayas and refuses to return it to PK. Bewildered, PK concludes that Tapasvi and other religious heads must be calling a "wrong number" to God and, as a result, spreading misunderstandings and meaningless rituals.
Meanwhile in Bruges, television reporter Jaggu (Anushka Sharma) falls in love with a man named Sarfaraz (Sushant Singh Rajput). Jaggu's father (Parikshit Sahni) objects to their relationship because Sarfaraz is a Muslim from Pakistan; he consults Tapasvi who predicts that Sarfaraz will betray Jaggu. Determined to prove them wrong, Jaggu proposes to Sarfaraz. She is heart-broken at the wedding chapel when she receives a letter calling off the marriage due to their differences. She returns to India where she is intrigued while watching PK distributing leaflets about a missing God. After hearing his story, Jaggu devises a plan to expose Tapasvi and recover PK's remote.
Jaggu encourages thousands of people to send in videos of their own experiences with religious heads calling "wrong numbers". However, during this period, PK contacts Bhairon Singh who says he will come to Delhi the next day. The next day, when PK goes to pick Bhairon very sadly as he found out Jaggu had a failed love, the station blasts killing Bhairon with nothing happening to PK. Later it is revealed that terrorist had planted the bomb to protect their faith in retaliation to PK's protests.
Finally, Tapasvi is forced to come into the studio and confront PK on-air. Tapasvi claims he has a direct connection to God and refers to his prediction of Sarfaraz's betrayal as proof. However, PK absorbs Jaggu's memories and discovers that Sarfaraz did not write the letter she received. Jaggu contacts the Pakistan Embassy in Belgium where Sarfaraz worked part-time; the embassy tells her that Sarfaraz still loves her and calls them daily to inquire whether she has called. Jaggu and Sarfaraz reconnect and Tapasvi, exposed as a fraud, is forced to return PK's device.
In the course of the film, PK falls in love with Jaggu but refrains from telling her because she loves Sarfaraz. He instead records tapes of her voice and fills his suitcases with batteries so that he can listen to the tapes on his home planet. While departing, he lies to Jaggu regarding the content of the tapes; Jaggu, despite knowing the truth, keeps quiet. She later publishes a book about PK and, at a book reading, claims that she misses him deeply.
The film ends with PK returning to Earth a year later on a new research mission with another alien (Ranbir Kapoor).
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