«Key Out of Time» by Andre Norton

Key Out of Time is the fourth novel in The Time Traders series by Andre Norton. It was first published in 1963, and as of 2012, had been reprinted in 17 editions with cover changes, as well as twice in a combined edition with The Defiant Agents. It is part of Norton's Forerunner universe.

Key Out of Time continues the series' premise, an encounter between Western heroes, and the Russian Communists, and the Baldies, a mysterious alien race that used time travel to alter Earth. Events in the previous novel, The Defiant Agents, are treated as a Time Agent failure - but read that novel for a different interpretation. The setting of this novel is a world, Hawaika, appearing to be a tropical paradise.

Time Agents Ross Murdock and Gordon Ashe return for another assignment, this time to the tropical island paradise planet of Hawaika.

Something apocalyptic has occurred in this planet's history, altering its topography and cleansing it of all intelligent life.

But something goes wrong when they attempt a peek at the past through a Time Gate, and with the Polynesian girl Karara and her trained dolphins Tino-rau and Taua, are plunged into a conflict between a dying super-race, barbarian natives, and invading aliens.

In her Time Trader novels Norton tacitly assumes that the physics of time travel differs so significantly from the physics of space travel, especially hyperdrive-propelled interstellar flight, that a civilization that discovers the technology of one simply will not discover the technology of the other. Earth's physicists have discovered the secret of time travel, but the engineers and scientists who built and use the time transporters have devised a clever way to obtain the secrets of space travel: if we can't discover the secrets for ourselves, we will get them from someone who did.