Can the Doctor's regeneration really be failing this time? It certainly looks that way as his two assistants, Tegan and Nyssa, help him recuperate on the tranquil planet of Castrovalva.

But are things as peaceful as they seem? What has happened to Adric? What is really going on in Castrovalva? And more importantly, where is the Master?

As the Doctor begins to recover he realises that the Master's trap is even more intricate than he could have imagined and that he will stop at nothing to gain his revenge over the Doctor.