All's quiet on Mercury Island after the events of GUARDIAN'S VOW, but Tane has not forgotten Gladiolus, her curse, or his role in breaking it.

But an old threat has arisen—the scientists responsible for the alterations done to Cirrus and Maria have returned. They are eager to recapture their old experiments ... and their descendants. They promise Gladiolus a breakthrough treatment for her blind eye, circumventing the curse. Accepting this treatment means Gladiolus must leave her old life behind, including Tane and the Atlantean Islands.

But the monster who laid the curse cannot be thwarted so easily, and accelerates Gladiolus's sickness in revenge for her choice. Now Tane must rescue Gladiolus from the curse, and his friends from the clutches of the scientists, and he'll need the full might of Mercury Island's superweapon to do it.