Old sailors have always feared the many horrors concealed by the tranquil looking waves of the briny depths. Now, the members of the TAC find themselves trapped between devils and the deep blue sea as they struggle to cope with the unexpected reappearance of Momiji's sister Kaede. Once believed to be dead but now known to be fighting on the side of the Aragami, Kaede's spell travels beyond the TAC's traditional urban turf to the surf swept beaches of Tono. There, while investigating the recent sightings of some very real sea monsters, Momiji finds her life turned upside down as the stories told by Kaede's friends and her own teammates cause her to question her own relative worth to both the TAC and Kusanagi. Then, Kunikida must face the most dreadful duty of all when an investigation into the death of an old comrade unravels a nightmarish secret of inhuman bloodlust and murder. The personal consequences of the war against the Aragami hit hard in this spectacular submarine edition of BLUE SEED!