Scientist Sanjay Mehra (Rakesh Roshan) creates a computer program from which he sends variations of the syllable om into space, hoping to attract the attention of extraterrestrial life. When he believes he has finally gotten a response, the scientific community mocks him. While driving home, an alien spacecraft appears overhead. Mehra veers off the road, causing the car to hit a stone, flip over, and explode. While Mehra dies, his pregnant wife Sonia (Rekha) survives the wreck and returns home to India. When their son Rohit (Hrithik Roshan) is born, he is mentally disabled due to the injury he received in the accident while still being a fetus. Sonia later learns from a doctor that although surgery is the only solution to Rohit's disability, it could potentially result in his paralysis or even death. His mother raises him in the town of Kasauli, where he grows up to be a man of limited intellect and childish personality, but with a loving and emotional heart. Rohit has a number of much younger friends: loyal children who play with him.
Nisha (Preity Zinta), a young lady comes to Kasauli and is initially antagonistic to Rohit due to his childish pranks against her. This causes her suitor, Raj (Rajat Bedi), and his friends to constantly harass Rohit, resulting in them breaking his scooter. Later, Nisha is sympathetic towards him and humors him, knowing about his mental disability from his mother, who chastises Nisha and Raj for assaulting Rohit and breaking his scooter. She invites Rohit to her house and introduces him to her parents, who are also sympathetic to Rohit and humor his childish interests.
Rohit and Nisha, now good friends, find Sanjay's old 'om' computer and inadvertently summon the aliens. The visiting aliens leave in haste, leaving one behind by accident. Rohit, Nisha and Rohit's friends find and befriend the alien, naming him Jadoo ("Magic") when they discover his psychokinetic abilities. Raj does not like Nisha's friendship with Rohit and spreads a fake rumour that he and Nisha are going to get married. This does not bode well with Nisha, who becomes upset with Raj. Rohit, however, is hurt as he was under the impression that Nisha and he were 'boyfriend and girlfriend'.
Jadoo discovers that Rohit is mentally abnormal so he uses his special powers, derived from sunlight, to enhance Rohit's mind. The next morning, Rohit finds himself independent of wearing glasses and has clear vision; he later solves a 10th standard mathematics problem, surprising his mathematics teacher and principal immensely. He later lectures his computer teacher for underestimating and insulting him and to understand the responsibilities of being a computer teacher. As a result, Rohit finds himself accepted by most of his peers and other students at the school.
Due to Jadoo, Rohit also becomes good at dance and sport, due to his physical strength and agility being increased to superhuman levels. Raj and his friends attempt to embarrass Rohit and his little friends by challenging them to a basketball match, although are surprised when Rohit uses his new physique to score several baskets at the beginning of the match. Raj and his gang become desperate and begin to play dirty; this causes the game to turn to their favor. After the Sun comes out, Jadoo uses the sunlight to help Rohit and his young friends in the basketball game with humorous results and Rohit's group eventually win the game when Rohit scores the match point in under 10 seconds, greatly amazing the judges and announcer.
Nisha and Rohit spend more time together, and Nisha begins to see Rohit in a romantic light. Rohit later proposes to her and she accepts. Jadoo's presence is kept a secret from everyone except Rohit, Nisha, Rohit's friends and mother.
Raj and his friends later corner Rohit's friends on their supernatural performance at the basketball game. Rohit's friends are forced to flee and Jadoo accidentally falls from their grip. Inspector Sukhwani, (Johnny Lever) sees Jadoo in his bag and informs the other policemen of him. The policemen capture the bag, although Rohit arrives in time to rescue Jadoo, leaving the Police behind. He is later confronted by Raj and his buddies. Rohit, now infuriated by the bullies, easily takes down Raj and his buddies but not before the police catch up to him. It is then revealed that Jadoo was not in the bag and had escaped when Sukhwani was busy calling the Police.
However, the Policemen are suspicious of Rohit and confront him in his house. They take Jadoo and knock Rohit out. When he regains conscience, Rohit uses his superhuman speed to catch up to the police cars and arrives just in time to save Jadoo from being taken to America by aeroplane. The spaceship, which he had summoned with his father's computer beforehand, arrives back and Rohit bids farewell to Jadoo. As Jadoo leaves, Rohit loses the physical and mental powers bestowed upon him and reverts to his childish, disabled self, which saves him from prosecution by the Indian government. After everything dies down, Raj and his buddies return to harass Rohit, causing the latter to kick Raj's ball into the bully's face with astounding force as Jadoo returns Rohit's powers permanently. Rohit and Nisha thank Jadoo and live a peaceful life there after.Hrithik Roshan as Rohit Mehra
Rekha as Sonia Mehra
Preity Zinta as Nisha
Prem Chopra as Harbans Saxena
Rajat Bedi as Raj Saxena
Johnny Lever as Chelaram Sukhwani
Mukesh Rishi as Inspector Khurshid Khan
Mithilesh Chaturvedi as Computer teacher
Anuj Pandit, Mohit Makkad, Jai Choksi, Omkar Purohit, Hansika Motwani and Pranita Bishnoi as "The Super Six"
Nikhil Gaurav as a Policeman
Rakesh Roshan as Sanjay Mehra (special appearance)
Beena Banerjee as Indu
Indravadan Purohit as 'Jaadoo (Alien)'
The film's soundtrack was first released in June 2003 by Saregama. The music was composed by Rajesh Roshan, and the lyrics were by Dev Kohli. It also uses uncredited music from the Art of Noise ("Moments in Love") and Kraftwerk ("Trans-Europe Express"). The songs were immensely popular in the year. All the songs were copied from different albums from different nations. According to the Indian trade website Box Office India, with around 21,00,000 units sold, this film's soundtrack album was the year's fourth highest-selling.
Koi... Mil Gaya received highly positive reviews from the critics and was declared a "Blockbuster" by Indian Box office. According to Rotten Tomatoes, the film received 75% 'Certified Fresh'. Taran Adarsh of Bollywood Hungama noted, "Hrithik Roshan dominates the show and packs in a power-pack performance. The role of a mentally challenged person is no cakewalk, but the actor takes to it like a fish takes to water. He manages to pull off the zero to hero routine exceptionally well. As an actor, he scales dizzier heights with this splendid performance".