A mysterious disturbance in the hyperspacial vortex causes the Doctor and Sam to materialise the TARDIS in deep space.  Here they find that a huge derelict alien craft has become the subject of a dangerous confrontation between starships from the rival systems of Nimos and Emindar.  At the centre of the dispute is a ruthless politician with secrets of his own - a man who is willing to risk innocent lives to claim the derelict for himself.

While the Doctor and Sam find themselves accompanying an expedition into the heart of the alien vessel, strange and frightening incidents spread terror through the watching ships.  But exploring deeper into the derelict disturbs a sinister presence and takes the expedition towards an inevitable confrontation with fate - and perhaps beyond the boundaries of life itself...