The Doctor, Jamie and Zoe become victims of piracy when they materialise on a space beacon, minutes before it is blown to pieces. So begins their quest to be reunited with the TARDIS, whilst treading perilously across the paths of the Interstella Space Corps and a gang of murderous bandits. The eccentric prospector Milo Clancey gives the travellers passage in his ageing spaceship, but the old man is himself the focus of Space Corps investigations. Could he possibly be behind the destruction and salvage of so many beacons in the sector? What is the position of Madeleine Issigri, who runs her father's mining corporation on the planet Ta — and how might a locked room provide the answer to this mystery?