After plummeting from the Pharos Project telescope, the Doctor regenerates. The time travelers escape the Master's clutches, but the Doctor's regeneration is failing, and Adric goes missing in the depths of the TARDIS. Only Nyssa and Tegan can save the day by steering the TARDIS to the city of Castrovalva, renowned for its healing powers. But evil lurks at the heart of this simple paradise. (Episodes 1-4, 96 mins)