Naani movie is a all about an 8-year-old kid Nani (Mahesh Babu). He is fed up with the bad treatment meted to him by his mother. When he attempts suicide, he is rescued by a weird scientist 'Science' (Raghuvaran). Nani tells him that he is fed up with everybody harassing him in his house. Science offers him a service that would make 8-year old Nani into a 28 year old grown-up. Nani accepts the proposal. Nani becomes big and starts living outside. His mother lodges a complaint in police station that her son is missing.
Nani grabs a job in a Toy manufacturing company as a product manager. The nature of his job is to play with all the toys and suggest improvements. He earns quite a handsome amount. In the process, he interacts with Priya (Amisha Patel), the daughter of TOY company's chief (Nazar). Priya is madly in love with Nani. Later on Nani realizes that his mother loves him and she is missing him a lot. By looking at the sorry plight of his mother after he became big, Nani asks Science to revert him back as kid. When he makes Nani back to 8-year-old kid, there is a complication. Nani is an 8-year old kid in the day time and a 28-year old grownup during the night time. The rest of the film is all about how Nani manages it.