Soon after Cruz, Sailor, Emmett, and Lani return for their second year at Explorer Academy, their sense of calm is shattered when Sailor's older sister goes missing. Amidst incredible expeditions to far-flung destinations and competitive classes, Sailor bravely steps into the lead to discover how her sister's sudden disappearance may be linked to an illegal and deadly animal-trafficking ring.

Despite winning the prestigious North Star Award and being surrounded by close friends and teachers who she has traveled the world with, Sailor's never felt less sure of herself. As the recruits face puzzling obstacles and try to follow complex clues, Sailor realizes there's only one thing that could save her sister in time: revealing her own deepest secret.