The story begins at Sir Richard's Mental Sanatorium, run by the strict and authoritative Dr. Khurana (Om Puri), along with the other two main doctors, Dr. Sunil (Jackie Shroff) and Dr. Tanvi Khurana (Kareena Kapoor), who is the daughter of Dr. Khurana. Dr. Tanvi has treated a female patient (Sulabha Arya) who wears no. 36 with love and proper care, rebelling against Dr Khurana's inhumane ways of dealing with mentally ill patients. As she gets discharged, Tanvi doesn't get acknowledged for her efforts, which devastate her and since then, has become embittered and vows to never come close to any patient, especially no. 36.

Anand (Salman Khan) is brought to the mental sanatorium by a court order. His elder brother requests the doctors to admit him immediately. However, they insist on evaluating his mental condition before taking any decision. On being asked a few questions, Anand does not show any signs of insanity, and the doctors conclude that there is nothing wrong with him. Just then, Anand spots a housefly on the table. He tries to capture it but it evades him. While Anand's brother insists with the doctors, Anand suddenly becomes very violent, picking up a club and using it to hit everything the fly lands on. This convinces the doctors that he is indeed insane, and they finally admit him.

In the sanatorium, Anand tries to makes friends with other patients. Dr. Sunil develops a caring bond with Anand, which is revealed that Sunil was made a doctor by Anand's father (Firdaus Mavawalla). Anand makes a commotion almost everyday and acts childish which upsets Tanvi and demands that Anand be thrown out from the asylum.

A flashback ensues when Tanvi finds out about Anand's sad past. Anand was a musician in college who was deeply in love with Maya (Rimi Sen) in Romania. Maya was a nun and did not reciprocate her love towards Anand, until he threatens to commit suicide on the rooftop of the church by pretending to set fire on himself. Therefore, Maya ran and confessed her love towards Anand and they plan to get married. Maya throws a series of pranks on Anand, such as pretending to not ride a horse, when she can ride perfectly and driving the car perfectly, albeit in a dangerous manner, when she said she couldn't drive. Anand desires to get back at her and pranks by throwing her into a swimming pool, thinking she can swim but cannot. Afterwards, Maya drowns and Anand's life is completely shattered. He becomes traumatized, blaming himself for Maya's death.

After finding out, she feels guilty for her treatment towards Anand and apologizes to him. They become good friends, spend time together, and she and Sunil work together to cure Anand and make him remember his past. This works, and Anand completely gets over the trauma of Maya's death and is finally cured. He decides to leave the asylum but stays when Tanvi expresses her love to him. He reciprocates her love and the two are happy with each other. But fate plays a part in the story, Tanvi has already fallen in love with Anand, but is engaged to Karan (Sunil Shetty), under her father, Dr Khurana's approval.

When Dr. Khurana finds out about her and Anand, he is livid as he sees this as a doctor-patient relationship, not a genuine relationship from a father's point of view. He demands Tanvi to leave him but she refuses to back down. When Karan finds out, he tells Tanvi to go with Anand. Sunil tries to make Dr. Khurana see that mental asylum patients should be treated with love, care and affection, but Khurana stubbornly believes that patients should be treated with oppression and brutality. Anand rebels against the strict hospital regime and Khurana's disapproval of Tanvi and Anand, but he refuses to back down. This makes Anand explode into a violent rage against Dr. Khurana, which results in Khurana lobotomizing him. Seeing no way out, Sunil and Tanvi arrive at the asylum to smuggle Anand out so that he and Tanvi can run away together, but it is too late when Sunil finds out about Anand's lobotomy. Realizing he is better off dead, Sunil sorrowfully kills his neurologically-disabled friend by suffocating him with a pillow. After Sunil confesses to Khurana about the act of euthanasia, Tanvi becomes insane due to the shock of Anand's death. She is admitted as a patient in the same asylum and the same number Anand had, no. 36. Dr. Khurana is shown to be feeling guilty of losing his own daughter to a patient in the same asylum.

• Salman Khan as Anand
• Jackie Shroff as Dr. Sunil
• Kareena Kapoor as Dr. Tanvi Khurana
• Suniel Shetty as Karan
• Rimi Sen as Maya
• Om Puri as Dr. Khurana
• Manoj Joshi as P. K. Narayan
• Anil Dhawan as Deepak
• Asrani as Asylum patient
• Arun Bakshi as Asylum patient
• Javed Khan as an asylum patient
• Kurush Deboo as Munna a.k.a. Idiot
• Atul Parchure
• Nagesh Bhonsle
• Shaurya Chauhan

Kyon Ki released on 2 November 2005 to coincide with the festival of Diwali in India. It performed poorly at the box office and grossed over ?231 million. Another Priyadarshan-directed film was released on the same day, the comedy Garam Masala which was commercially successful at the box office, grossing over ?546 million.

The film received generally negative reviews from critics. Urvashi Asharl of The Times of India commented that the film was "a huge disappointment" as it had a predictable and dull plot with too many songs but praised the performances of Salman Khan, Kareena Kapoor, and Jackie Shroff. Rediff.com reviewer Patcy N. also felt that there were too many songs and criticised the script and slow pacing of the film. Namrata Joshi writing for the magazine Outlook wrote that the central romance between Khan and Kapoor was not believable and criticised the acting and script saying "all the actors just sleepwalk through their ill-sketched roles". She also described the depiction of violence against mentally ill patients as therapy in the film as "offensive to say the least".^[7]

A group of psychiatrists in Mumbai referred the film to the National Human Rights Commission of India asking for certain scenes to be removed. They felt that the film's depiction of mentally ill patients and their treatment was derogatory and misleading.^[8][9]

Himesh Reshammiya composed all the songs while S. P. Venkatesh had composed the film score. The lyrics were penned by Sameer. According to the Indian trade website Box Office India, with around 13,00,000 units sold, this film's soundtrack album was the year's fourteenth highest-selling.^[10]

Some of the scenes were filmed in Romania.^[11]