Ayumu Tamari is afflicted with a disorder called Crystallization Syndrome. When he experiences intense stress, sections of his body harden into crystal and become almost impossible to move. His crippling social anxiety also stops him from speaking with anyone in his class, and the frequent crystallization forces him to repeat a grade at high school. The only comfort he finds is his homeroom teacher, Kouya Onihara, whom he calls Oni-chan Sensei. Kouya is an avid crystal collector and researcher, and he admires the beauty of Ayumu"s crystalline form. The added pressure from school and the taboo nature of their student‑teacher bond make it hard for Ayumu to live a normal life, as he battles his feelings and tries to avert full crystallization.
