The Curse of Peladon

The Doctor and Jo simply meant to take a test ride in the TARDIS to check that it was working. But instead the TARDIS takes them to Peladon, a medieval world which is seeking membership of the Galactic Federation. Delegates have gathered on the planet from all over the galaxy, and among them are representatives from Alpha Centauri, Arcturus, and Mars, some of whom are the Doctor's oldest and most dangerous enemies.