The Doctor's latest regeneration has proven more unstable than his previous ones. His two assistants, Tegan and Nyssa, help him recuperate on the tranquil planet of Castrovalva. But are things as peaceful as they seem on Castrovalva? What has happened to Adric? And more importantly, where is the Master? As the Doctor begins to recover he realises that the Master has laid a trap even more intricate than he could have imagined and that he will stop at nothing to gain his revenge over the Doctor.