The Doctor, Romana and K-9 continue their search for the six disguised segments that make up the powerful Key to Time. When the TARDIS lands on the seemingly peaceful planet of Tara, the Doctor claims he is owed a holiday. While he takes time off to go fishing, Romana quickly locates the Fourth Segment. All is going well until the time-travellers are drawn into the complex political intrigues of Tara. The benevolent Prince Reynart is soon to be crowned king, but the wicked Count Grendel wants both the throne, and Reynart's bride-to-be, the Princess Strella, for himself. Grendel plots with the android builder Madame Lamia, and soon both Reynart and Strella are replaced by robotic duplicates. Unfortunately for Romana, she also bears a startling resemblance to the princess - which makes her enormously valuable to the Count. With time running out, the Doctor is unwillingly thrust into the roles of android repair-man, king-maker and, ultimately, swash-buckling hero, when he finds himself duelling with Grendel, famous for being the finest swordsman on Tara...