The Return to Earth — Margaret Gregory

Even before the war on Tymorea, the Elders foresaw that Great Ones Tymos and Kryslie would have an imperative mission on Earth. But as the Tymoreans prepare to build an Earthbase to support them, they discover that specifications for two vital protective shields are missing. Tymos and Kryslie must find the work of a missing Tymorean scientist and build the generator before the base is found. Then, as the missionaries are transported to Earth, the Guardians of Peace step in and send Tymos and Kryslie back in time.