Humans are colonizing the area, and a rival group sets up on Janus Prime via a mysterious transmit system left behind by the planets' former inhabitants. But what is its true purpose?
When the Doctor and Sam arrive they must piece together a centuries-old puzzle. How can Janus Prime's moon weigh billions of tons more than it should? What is the secret purpose of the hyperspatial link? They discover a terrible weapon is hidden in the glowing sands of the planet, one that if it falls into the hands of the warring humans could destroy the galaxy.