The Ragged World — Judith Moffett

In the early years of the twenty-first century, Earth teetered on the brink of ecological destruction. Then the alien Hefn came, determined to save the dying Earth - and to the Hefn, the ends always justified the means. Humans were given nine years to correct their mistakes - alone, with no recourse to the Hefn's advanced technology. If by then the Earth's ecology had not stabilized, the Hefn would solve the problem for good . . . by eliminating humans entirely.

But slowly, against their will, some of the Hefn became deeply involved with their human counterparts. And to the handful of people who came to know them, the Hefn made a great difference: as mentors, researchers, rulers . . . and saviors. But could those few friendships sway the Hefn to help save a despoiled planet - and the human race?