he Doctor and Jo take the TARDIS on a test flight to the planet of Peladon, a planet which is attending negotiations to join the Federation, although not everyone is in favour, and someone is using the spirit of Aggedor in an attempt to drive a wedge between Peladon and the Federation.
The TARDIS falls down the side of a mountain, and the Doctor and Jo seek shelter inside the citadel where they are mistaken as the Earth delegates. Someone attacks Arcturus, and the Doctor is accused. He has to face the King's Champion, and survives.
At first the Doctor suspects that the Ice Warriors are to blame, but they really do appear to be pacifists. Eventually it emerges that High Priest Hepesh and Arcturus are to blame. Arcturus has convinced Hepesh that the Federation would exploit the planet of its mineral wealth, although this was Arcturus's intention. Arcturus kills the King's Champion, and is in turn killed by one of the Ice Warriors.
The Doctor finds that Aggedor isn't such a beast, but is a rather more timid creature. Hepesh has been using the creature and orders it to kill the Doctor. Instead it turns on Hepesh.