"He is only a kid…" is something that is heard ever too often. But when a kid displays the valor and wit that is no child's play and the presence of mind that belies his age, it certainly is no kidding matter. Hari Puttar is the awe-inspiring story of one such kid. Hari Prasad Dhoonda (Zain Khan) or Hari Puttar, as he is fondly called, is a bright 10 year old who has recently moved from India to the U.K. with his parents (Sarika and Zakir Khan). His father, Prof. Dhoonda is assigned to work on a secret project with the Defense Forces at a remote location in the U.K. He has access to confidential information that is stored on a chip and safely kept at his house.

Within days of their arrival in London, Hari's aunt (Lilette Dubey) and uncle DK (Jackie Shroff) visit the Dhoonda residence with their own as well DK's boss' children. Suddenly, the house is filled with the effervescence of no less than ten kids. At this point, everything goes for a spin for Hari Puttar. Not only is he thrown out of his room to accommodate the girls, he is also ragged and ridiculed by all the other kids for being too naïve. Hari Puttar suddenly feels unwanted, undermined, and unappreciated.

To top it, even as DK arranges a vacation for the entire family, one night before they take off, Hari's mother sends him off to the attic bedroom as a punishment for no big fault of his. As luck would have it, the next morning, the whole family leaves in a tearing hurry, forgetting that they have left something integral behind - Namely Hari and his little cousin Tuk Tuk (Swini Khara). But this realization does not dawn until they are halfway into their journey.

When Hari and Tuk Tuk discover that they have the large house to themselves, they begin to enjoy the sudden freedom.

Meantime, an infamous don, Kali Mirchi, assigns two bumbling burglars (Saurabh Shukla & Vijay Raaz) to steal the secret formula chip from Hari's house. Hari accidentally discovers the mission of the burglars and is determined to save the secret formula from being exposed.

The so called naïve Hari Puttar embarks on a journey of great adventure and suddenly discovers the self-confidence, creativity and gumption that he did not know he possessed…With his innovative plans , determined wit, and with able support from Tuk Tuk, Hari Puttar manages to not only save the secret chip but also captures the two unsuspecting thugs.

Hari's mother in the meanwhile, desperate to get back to Hari goes through her own adventure to reach back home… And when she does, she finds that she had left behind a son …and had come back to a little hero.