The Earthcore team's newfound fame means they're asked to help calm an earthquake cluster shaking the Hauraki Gulf yet again. No geological cause can be found - indicating a source related to the Taniwha of legend. Anira and her team travel to Waiheke Island, where clues point to the old, closed-up WWII fortification tunnels that honeycomb the island's remote eastern headland. As the quakes continue, the team investigates a mystery from the 1940s. To solve it, they’ll have to head underground, where a young member fumbles with dangerous emerging power and the rest of the team find their magic almost useless. Split up and trapped beneath the surface, Earthcore races to resolve a wartime injustice, but the Taniwha are angry - and the tide is rising.