Aria lives on a well-ordered planet whose people have eradicated illness and even death. Earth is their 'shadow planet' which they populated with humans centuries ago so they could study them and learn from their experiences.
Now the experiment is coming to an end and Aria must go to Earth with her scientist father to set off a train of events which will destroy its people. Brought up to believe that humans are inferior, Aria is shocked to discover that she is herself half human, and amazed to find that Earth-dwellers live life to the full and feel love for each other, even though they are mortal.
But once she understands this, how can she save them, and herself, from destruction?