Key Out of Time

In the present day, Time Agents Ross and Gordon come with settlers to the water-dominated planet, Hawaika, to search remains of the alien Baldies from the distant past. Local intelligent dolphins assist them. While investigating, a time gate fails, destroying itself and stranding them widely in the unknown past. The native dolphins and humans can communicate, and Ross learns Gordon is hostage in a castle through a native, Loketh.
Ross and Loketh are captured by seafaring Rovers, then join them. They recover a Rover island captured by the Baldies. Ross convinces a coalition of natives the Baldies are playing them against one another. Ross finds Ashe at last, in a castle the company of the mystic and advanced Foanna, who turn out to be only three, the last of their race. The Foanna set a trap for the Baldies, using their assets as bait, but they cannot win against the whole force without increasing their numbers.
Ross and Ashe agree to a process mentally joining them with the Foanna. A second encounter with the Baldies, they win. In a final encounter, Ross is teleported to a Baldy ship like the one familiar to him from Galactic Derelict, and sets its course to a random destination. The main Baldy installation is simultaneously attacked and the Baldies driven off the planet.