Scientist Sanjay Mehra creates a computer 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 received a response, the scientific community mocks him. While driving home, an alien spacecraft appears overhead. Distracted, Sanjay drives off the road, causing the car to flip over and explode, killing him.
His pregnant wife Sonia survives the wreck and returns home to India. When their son Rohit Mehra (Hrithik Roshan) is born, he is mentally disabled due to an injury he received in the accident while still a foetus. Sonia learns that although surgery is the only solution to Rohit's disability, it could result in his paralysis or death. His mother raises him in the town of Kasauli. He grows up to be a man of limited intellect and a childish personality, but with a loving heart. Rohit also has several 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 friend, Raj (Rajat Bedi), and his friends to continuously harass Rohit, resulting in them attacking him and breaking his scooter. Later, Nisha learns about Rohit's mental disability from his mother, who chastises Nisha and Raj for assaulting Rohit and breaking his scooter. Nisha grows sympathetic towards Rohit after witnessing his daily struggles and introduces him to her parents. They also become sympathetic to Rohit and care for 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 getting married. Nisha is angered by Raj's lies and breaks her friendship with him. Rohit, however, is hurt as he was under the impression that he and Nisha were boyfriend and girlfriend.
Jadoo discovers that Rohit is mentally abnormal and uses his powers, derived from sunlight, to enhance Rohit's mind. The next morning, Rohit finds himself with a clear vision; he later solves a 10th standard mathematics problem, surprising his mathematics teacher and principal. He then lectures his computer teacher for underestimating him and for not understanding the responsibilities of being a teacher. As a result, Rohit finds himself accepted by most of his peers and other students at the school.
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 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 game. Raj and his gang become desperate and begin to play dirty; this causes the game to turn to their favour. After the sun comes out, Jadoo uses the sunlight to help Rohit and his young friends win the game. 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.
Later, Raj and his friends 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 about him. The policemen capture the bag, although Rohit arrives in time to rescue Jadoo, leaving the police behind. Raj and his buddies later confront him. Rohit, now infuriated by the bullies, quickly 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 police are suspicious of Rohit and confront him in his house. They take Jadoo and knock Rohit out. When he regains consciousness, 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 and Rohit bids farewell to Jadoo.
As Jadoo leaves, Rohit loses the physical and mental powers bestowed upon him and reverts to his, 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 as Jadoo returns Rohit's powers permanently. Rohit and Nisha thank Jadoo and live a peaceful life thereafter.
