Akira Tsubaki is a normal high schooler who sits next to new transfer student Mikoto Urabe during classes. Her face half hidden by hair, Urabe earned a reputation as a weirdo on her first day of school when she started laughing loudly for no apparent reason in the middle of a class. She does not like to interact with her classmates and instead prefers to nap on her desk. One day after school, Tsubaki finds Urabe sleeping on her desk and wakes her up. However, after Urabe goes home, he notices her drool on her desk and impulsively tastes it. A few days later, he is struck down by a fever and is visited by Urabe at his home. After confirming the fact that Tsubaki tasted her drool that day, she informs him that his fever is nothing but love sickness and that he is suffering from withdrawal symptoms of her drool. Thus, the two enter into a strange relationship where Urabe feeds Tsubaki her drool everyday to prevent any more sickness. [Written by MAL Rewrite]

Nazo no Kanojo X was published in English as Mysterious Girlfriend X by Vertical Inc. under the Vertical Comics imprint in 2-in-1 omnibuses from March 15, 2016 to June 13, 2017. The series was digitally published in English by Crunchyroll.