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.