The Prion star system features two habitable planets: Zolfa-Thura, a desert world, and Tigella, a jungle world. Tigella society is based on two castes: the scientific Savants, led by Deedrix (Crawford Logan) and the religious Deons, led by Lexa (Jacqueline Hill, who played the First Doctor’s companion Barbara Wright). The Deons worship the Dodecahedron, a 12-sided crystal given by the god Ti. The Savants are using it as a power source for the entire civilization. The planet leader, Zastor (Edward Underdown), mediates between the two factions. Zastor invites his old friend the Doctor to help, particularly as the energy source is beginning to fluctuate. When the Doctor, Romana, and K9 try to land the TARDIS on Tigella, they are trapped in a chronic hysteresis, or a time-bubble, by Meglos, the last surviving Zolfa-Thuran, a cactus-like xerophyte creature who wants to steal the dodecahedron, which came from Zolfa-Thura in the first place. He is served by space-pirates called Gaztaks and an Earthling kidnapped by the pirates, who can help Meglos take on human form. The shape he wants to shift into is the trapped Doctor. In this form, he infiltrates Tigella and greets leader Zastor as an old friend, who asks him to examine the dodecahedron. The Doctor and Romana escape the time-bubble and land in the jungle. The Doctor heads off to see Zastor and Romana gives the pirates the slip, then heads for the city herself. Meglos, as the Doctor, shrinks the dodecahedron to tiny size and steals it. However, the Earthling inside him struggles to emerge, sometimes causing cactus spikes to break out on Meglos’ body. The real Doctor arrives and, naturally, is accused of stealing the dodecahedron. As the power levels in the city begin to drop, Lexa uses the situation to arrest Zastor and Deedrix, expel other Savants, and sacrifice the Doctor to the great god Ti.
The doors of the city are sealed. The Earthling tries to free himself again, and a Savant named Caris (Colette Gleeson) takes on Meglos in his Doctor shape, but Romana thinks she is fighting the Doctor and overpowers Caris, letting Meglos escape and reunite with the pirates, escaping to Zolfa-Thura with the miniaturized dodecahedron. The real Doctor’s innocence is proven. Lexa is killed by a Gaztak pirate. The Doctor, Romana, Caris, Deedrix, and K9 head for the TARDIS.
Meglos’ ship lands on Zolfa-Thura and Meglos restores the dodecahedron to full size and uses it to power a weapon against Tigella. When the Doctor arrives, he impersonates Meglos, confusing the Gaztaks so much he is able to re-direct the weapon at Zolfa-Thura. Meglos drops the Earthling and appears as his own cactus self. He is unable to prevent the Doctor and his posse from escaping in the TARDIS. Zolfa-Thura is destroyed. Caris and Deedrix begin to rebuild Tigellan society. The Doctor and Romana leave in the TARDIS to take the Earthling home, but there is an urgent call from the Time-Lords.
Meglos is just a voice out of a cactus and has no depth or character, except when Tom Baker plays him, looking strange indeed with cactus-spikes coming out of his face and hands. Romana is witty and cunning and a perfect foil for the Doctor. Madame Tussaud’s displayed wax figures of both the Doctor and his xerophyte doppelganger at the same time, the only time anyone appeared twice in the same museum. Two of the Gaztaks were Lieutenant Brotodac (Frederick Treves), and General Grugger (Bill Fraser), mostly played for laughs. Brotodac is an anagram of Bad Actor, and Bill Fraser took on the role of Grugger providing he could kick K9, which he did.