In a citadel on the planet Peladon, Chancellor Torbis (Henry Gilbert) reports to King Peladon (David Troughton) that the delegate from Alpha Centauri has arrived. Peladon wants the planet to join the Galactic Federation but Hepesh (Geoffrey Toone), his High Priest, is against such a move. Peladon has invited a committee of assessment to the planet to decide on the issue and all are now present, barring the chairman delegate from Earth. Torbis leaves to inform the other delegates of Alpha Centauri's arrival but he is attacked and killed by Aggedor (Nick Hobbs), the sacred and mythical beast of Peladon. 

The TARDIS arrives on the slopes of the mountain on which the citadel is perched, and when the Doctor and Jo exit, it tumbles to the bottom of the mountain. Unable to descend, the Doctor and Jo struggle up the mountainside through rain and wind until Jo finds a cave which leads to a series of underground tunnels under the citadel. They find their way into the citadel and see an Ice Warrior lumber past just before they are caught and taken to the throne room. 

There, they meet Peladon and the assembled delegates: Alpha Centauri (Stuart Fell; voice: Ysanne Churchman), Arcturus (Murphy Grumbar; voice: Terry Bale) and Martians Lord Izlyr (Alan Bennion) and his aide Ssorg (Sonny Caldinez). Izlyr assumes that the Doctor is the delegate from Earth and the others follow suit. The Doctor, not wishing to cause undue trouble, also plays along, introducing Jo as Princess Josephine of TARDIS. Peladon is very taken with Jo. While they are talking, Hepesh gestures to Grun (Gordon St. Clair), the King's champion, and he slips out of the throne room and positions himself to topple a statue of Aggedor onto the delegates as they leave the throne room. 

Luckily, the Doctor sees the statue falling and pushes the delegates out of its path, saving their lives. Grun creeps back into the throne room and while Peladon tries to assure everyone that the planet is safe, Jo slips away and finds a piece of electronic equipment up by where the statue was originally standing. Later, the Doctor confirms that it is an electronic key made from trisilicate, which can be found only on Mars, home of the Ice Warriors. 

Arcturus is attacked and the Doctor manages to re-route his circuits and stabilise his condition. Someone has taken a servo junction unit from his life support systems. Jo goes investigating and finds the missing unit in the Warriors' room. She is discovered by Ssorg, but escapes from the room by climbing out of the window and edging round to another room. She sees Aggedor in the corridors and, panicking, runs straight into Izlyr and Ssorg. She talks with the Martians and realises that they are innocent. 

The Doctor has been lured into the tunnels under the citadel by Grun and is chased by Aggedor. He arrives at the sacred temple of Aggedor and Hepesh has him arrested for sacrilege, a crime for which there is only one punishment à death. Jo appeals to Peladon and he commutes the sentence to a battle to the death with Grun. 

Hepesh offers to help the Doctor escape and gives him a map of the tunnels. He tells him that he does not stand alone in his desire to prevent the planet from joining the Federation. The Doctor fits a mirror to the top of his sonic screwdriver and leaves. In the tunnels, the map leads him to Aggedor. The Doctor hypnotises the beast with the spinning mirror and by crooning a Venusian lullaby. Jo, who has followed the Doctor, frightens Aggedor off with a flaming torch, and they return to tell Peladon that Aggedor is real. He does not believe them and the trial by combat takes place. 

The Doctor wins out over Grun, but refuses to kill him. Arcturus points his laser weapon at the Doctor, but Ssorg destroys Arcturus before he can fire. Hepesh escapes into the tunnels and recruits the guard captain (George Giles) and some of the guards (Stewart Barry, Derek Chafer, Billy Dean, Royston Farrell, Mike Stevens, Chris Webb, Peter Brace, Bill Horrigan, Mike Horsburgh, Dinny Powell, Roy Street, Rocky Taylor, Terry Walsh) to fight against Peladon. The delegates discover that all their communicators have been destroyed and they are unable to summon help. The Doctor decides to find Aggedor. 

Hepesh and his guards storm the throne room and fight with Peladon's guards. The Doctor hypnotises Aggedor and brings him to the throne room where he attacks Hepesh and kills him. With this, the guards surrender. 

The Doctor and Jo find the TARDIS in one of the citadel rooms where it has been placed by Peladon's men. The Doctor thinks that the Time Lords may have brought them here at this time to help out. Peladon's coronation is about to take place and the Doctor and Jo are looking forward to it. However, the real delegate from Earth (Wendy Danvers), arrives and the Doctor and Jo are forced to make a swift exit.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}