Peaceful aliens, the Kyterians, discover an evil foe determined to wipe out all other intelligent races. Faced with extinction the Kyterians reach out to Earth in the 13th Century for help in battling their implacable foes, the Darlo. Those recruited for the struggle do not themselves engage in the war against the Darlo, who have already exterminated numerous civilisations. Instead their descendants are educated and trained to become full members of their adopted society and eventually to serve as members of their combat forces. When they finally fight the Darlo a grand adventure filled with dramatic surprises begins.