Harmful Insect follows a girl named Sachiko after her father dies, her mother attempts suicide, and her entire class is gossiping about her possibly having an affair with her sixth grade teacher who recently moved away. Because of all this, Sachiko stops going to school and hangs out with a street-dweller named Takao and his mentally disabled friend. When Takao is beaten to death Sachiko is forced to return to her former life, only to face more unwanted trauma at the hands of adults who never seem to have her best interests in mind.
Mitamura Yuka (Hiroko Yakushimaru) is a normal shy middle school student that has psychic powers. When a new student with similar powers begins to show his skills, by stealing the student government election, Yuka and her friends vow to stop him. The election has uncovered something which will put Yuka's powers to the ultimate test!