The crystal Kingdom of Chalcedonia was once a beautiful place, under the rule of the Princess Rosalia, they lived happy peaceful lives. Until one day, monsters called Nightmares destroyed the kingdom. Princess Rosalia and her fairy knight Diana hid deep in the castle, praying that their last hope, the magical Rose Crystal Mirror would produce a miracle, and save their home, however, they were attacked by the leader of the Nightmare Agency, Druzy. Diana fought helplessly before finding herself transported Earth, but her Princess was nowhere to be seen.

Unfortunately, the nightmares began to attack this new world too. Valerie Amaranth, an ordinary 16-year-old, has her world changed forever when she receives the powers of the Legendary Crystal Warrior Diamond Heart. Now with the help of Diana, she must find her allies, defeat the Nightmares, and rescue the missing princess Rosalia.