John Christopher   As they walk through a park in the distant future, Harl and Ellen talk about their work and their lives. But they will never have a life together because their work as scientists is more important to them than their love. Harl plans to leave Earth, on a long and dangerous journey through space. Ellen plans to stay on Earth, to change the way the human mind works. When Harl returns to Earth, Ellen will be long dead . . . and the world will be a very different place.