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.
A human-looking alien from a highly advanced but emotionless all-male society is sent to Earth to impregnate a woman and bring the child back to their planet.
When the Master steals the Time Lords' secret file on the Doomsday Weapon, they grant the Doctor a temporary reprieve from his exile on Earth to deal with the crisis.
Searching for the third segment to the Key to Time brings the Doctor and Romana to present-day Earth, where the travellers have to contend with stone circles, Druidic rituals and a not-so-mythical goddess known as the Cailleach.
Captured in a time corridor, the Doctor and his companions are forced to land on 20th century Earth, diverted by the Doctor's oldest enemy - the Daleks.
As evidence for the defense at his ongoing trial, the Doctor presents an adventure from his future when he is travelling with a computer programmer named Mel.
The Doctor and his companions arrive on a spaceship headed for Earth, populated by natives of Earth from various different eras and commanded by the leaders of the Urbankan race.
Once upon a time there lived in the same village two men bearing the very same name. One of them chanced to possess four horses, the other had only one horse, so, by way of distinguishing them from each other, the proprietor of four horses was called "Great Claus," and he who owned but one horse was known as "Little Claus".
