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Trying to transport Tegan to Heathrow airport on present-day Earth, The TARDIS accidentally lands on-board a alien spaceship traveling to Earth which will arrive in 4 days. Where the Doctor, Adric, Nyssa and Tegan encounter the frog like Monarch, ruler of the doomed planet Urbanka. Only to find Monarch has abducted generations of humans from different cultures and converted them into cyborgs. The Doctor and Tegan discover Monarch's true goal is to travel faster than the speed of light, and traveling back into time where he will meet himself and wipe out the human race, repopulating the Earth with his own race.