The Doctor and Sarah find themselves in an asteroid, on the outer reaches of Earth’s solar system. It’s the future, and humans have colonised this inhospitable place, to mine vital minerals to supply their home planet. However, links with Earth are becoming ever more strained, and forces within the colony are seeking independence. Revolution is in the air.

Meanwhile, in the deepest chambers of the asteroid, a powerful force is emerging. Its influence is growing rapidly. But how will it affect Sarah Jane Smith and her place in future history?