Ryo Aihara, a shy girl who has trouble holding conversations, is the only one at her school without a cell phone. With no friends to text or call, she convinces herself that owning one is pointless. However, with a strong desire for a cell phone, she envisions an ideal one to every detail.

Despite it being a made-up device, Aihara receives a call and realizes that she is the only one who hears the ringtone. Apprehensive but curious, she answers the call and hears a high school boy on the other end of the line. He introduces himself as Shinya Nozaki and claims that he is also using a phone in his head. The bizarre circumstance leads to the development of a close friendship between Aihara and Nozaki, one in which they can confide in each other.

Note: Tsuzuki Setsuri also did a manga based on the same novel.