Two planets, Janus Prime and Menda, orbit a Red Giant on the edge of the galaxy. The planets lie diametrically opposite each other on either side of the huge sun—but where Menda is rich and fertile in the light of the sun, Janus Prime's moon leaves the sun in a constant state of eclipse. 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.