Doctor Who: The Twin Dilemma

The Doctor has regenerated and all is not well. Confused, angry and acting strangely, the new Doctor decides to become a hermit, so takes his concerned companion Peri to a desolate asteroid where he plans to live for several centuries. However, the time travellers soon uncover a plot that threatens the entire galaxy. Why has an alien Gastropod kidnapped a pair of genius twins, and will the Doctor recover in time to find out?