The Androids of Tara — David Fisher

In search of a segment of the Key of Time, the Doctor, Romana, and K9 arrive on the planet Tara, whose population has long mastered the art of android engineering. Romana is apprehended by the ambitious Count Grendel, who seeks to overthrow the king in waiting, and the Doctor is coerced into helping those loyal to the crown. With Reynart locked away in Grendel's dungeons, the Doctor must perfect an android that can be crowned in his place. Yet Reynart is not the only one of Grendel's prisoners to have a double: the captive Princess Strella is Romana's doppelganger. A deadly game of cat-and-mouse ensues as Grendel seeks to destroy the Doctor in his race to the Taran throne, x001A.