The passing of a member of the Astronomical Club served as the catalyst for everything. From this, the club's phantom supposedly sprang to life. In the Astronomical Club's abandoned observatory, grumpy insomniac Ganta Nakami meets the moderately rebellious but pleasant Isaki Magari. When Magari is startled awake, she tries to flee the area, but a shattered door prevents her from doing so, trapping the two within. The two develop an odd friendship, and the observatory serves as a safe haven for their interactions. In Kimi wa Houkago Insomnia, Nakami and Magari, who both suffer from insomnia, spend time in the observatory under the stars as they support one another and work through its long-lasting affects on student life.