Saya Hokage, a high school girl who is unable to sleep due to insomnia, encounters Hitsuji Konparu, a girl who can put anyone to sleep as a "lover" in a dream. When Hitsuji's senpai – Ran Aizome – sees potential in Saya, she ends up joining them and their group of Sleepwalkers. As it turns out, unbeknownst to the common citizens in their town, a battle has been unfolding between the Suiju – beings that possess people's spirits in the land of sleep – and the Sleepwalkers, who have the power to move about freely in their dreams.

Sleeping together as a team, Saya and her newfound group are doing a good job hunting Suiju. That is, until an unexpected darkness comes along...

Will the girls be able defend humanity's sleep?