"He's one of my own people, Victoria, and he's hunting me."

Darkheart: a faded neutron star surrounded by dead planets. But there is life on one of these icy rocks — the last enclave of the Earth Empire, frozen in the image of another time. As the rest of the galaxy enjoys the fruits of the fledgling Federation, these isolated imperials, bound to obey a forgotten ideal, harbour a dark obsession.

The Doctor, Jamie and Victoria arrive to find that the Federation has at last come to reintegrate this lost colony, whether they like it or not. But all is not well in the Federation camp: relations and allegiances are changing. The fierce Veltrochni — angered by the murder of their kinsmen — have an entirely different agenda. And someone else is manipulating the mission for his own mysterious reasons — another time traveller, a suave and assured master of his work.

The Doctor must uncover the terrible secret which brought the Empire to this desolate sector, and find the source of the strange power maintaining their society. But can a Time Lord, facing the ultimate temptation, control his own desires?