In order to protect the country's citizens, by using supernatural powers, the secret government esper research organization known as "Kokueken" was created. Here, it is where children with esper potentials are gathered, and they are trained into full fledged espers. Espers trained here are then secretly send into society where they must use their powers to help people and the world. An apprentice esper of this organization, Mano Erina, alias "Hanbun Esper (Half Esper)", is a girl full of curiosity. Naturally, if one can not become a full fledged esper, it is impossible to be sent into society. With her strong will, "I want to leave here, and go see the world!", using her esper powers, she escaped Kokueken's facility and disappaeared!
That's right! "Hanbun Esper (Half Esper)" Mano Erina can not fully control her own powers. She is a half trained esper with incredible power potentials. Fearing what kind of power could be unleashed from Mano Erina's emotions, it is giving the organization's research chief a big headache. As they seek her whereabouts...