15 - Key Out of Time by Andre Norton

Join Time Agents Ross Murdock and Gordon Ashe as they embark on another thrilling assignment, this time on the breathtaking tropical island planet of Hawaika. Once a paradise, its history has been marred by an apocalyptic event that reshaped its landscape and wiped out intelligent life. When their attempt to glimpse the past through a Time Gate goes awry, they find themselves in the midst of a chaotic struggle involving a dying super-race, fierce native tribes, and relentless alien invaders. Alongside the brave Polynesian girl Karara and her loyal dolphins Tino-rau and Taua, they must navigate this perilous conflict to uncover the secrets of Hawaika. (Summary by Mark Nelson)