In the near future, Tokyo was left flattened as a result from a great earthquake. A new city, MegaTokyo, was then recreated due in no small part from the aid of a multi-million dollar company, Genom Corp. Genom created and mass-produced biomechanical creatures called Boomers to aid in the restoration of MegaTokyo. When the Boomers began to run out of control, the ADPolice at first tried to stop them, but they proved to be far more difficult to deal with than was first imagined. Under the ever looming Boomer threat, a group of four girls from varying degrees of society banded together. Calling themselves The Knight Sabers, they were the only ones with enough firepower and resourcefullness to defend the fledgling MegaTokyo from Genom and its berserk Boomers.
In the future, personal computers have developed into "Persocoms", mobile computers that look like human beings. Hideki Motosuwa, a prep school student, desperately wants to buy a Persocom but cannot afford to. One day he finds a Persocom that has been thrown away and decides to keep it. When he turns on the Persocom, all she can say is "Chii" so he decides to name her that. After a while it starts to become apparent that Chii is more than an average Persocom. Having much better performance, it seems that Chii might be a "Chobit", a type of advanced Persocom rumored to have independent thought.