The TARDIS arrives on the planet Marinus on an island of glass surrounded by a sea of acid. The travellers are forced by the elderly Arbitan to retrieve four of the five operating keys to a machine called the Conscience of Marinus, of which he is the keeper. These have been hidden in different locations around the planet to prevent them falling into the hands of the evil Yartek and his Voord warriors, who plan to seize the machine and use its originally benevolent mind-influencing power for their own sinister purposes.
The woman who will become Catherine the Great marries into the Russian royal family when she weds Grand Duke Peter, the nephew of Empress Elizabeth. Although the couple has moments of contentment, Peter's cruel and erratic behavior causes a rift between him and Catherine. Mere months after Peter succeeds his aunt as the ruler of Russia, a revolt is brewing, and Catherine is poised to ascend to the throne as the country's new empress.
The adventures of The Doctor, a time-traveling humanoid alien known as a Time Lord. He explores the universe in his TARDIS, a sentient time-traveling spaceship. Its exterior appears as a blue British police box, which was a common sight in Britain in 1963 when the series first aired. Along with a succession of companions, The Doctor faces a variety of foes while working to save civilizations, help ordinary people, and right many wrongs.
