Vanderdeken's Children — Christopher Bulis

In attempting to discover where these vessels come from, the Doctor and Sam unearth a terrible truth. The aden ship is caught in a closed loop of time, being neither created nor destroyed, constantly circling the vortex. The Doctor wants the ship to be destroyed, but the Ximosian and Emindarans are caught in a wrestle for power, and both desire to possess the spacecraft and transform its power into a source for their own political ends.