Prince Reynart was the rightful ruler of Tara.
Count Grendel of Gracht wanted the throne and was expected to make a move during Reynart's coronation. To protect himself, Reynart had an android duplicate of himself built. Grendel kidnapped the prince so that he could not be crowned.
If he did not show up at the ceremony, Grendel would be crowned. The Fourth Doctor, Farrah and Zadek used the android in Reynart's place so that he would still be crowned. Reynart was joined in his cell by Romana I, who tended to his injuries. He was released after the count's defeat. (TV: The Androids of Tara)