The Prion star system contains two habitable planets which have supported civilisations: Zolfa-Thura, a desert world devoid seemingly of life structures bar five giant screens; and Tigella, a jungle world inhabited by the humanoid, white haired Tigellans. The structure of Tigellan society is based on two castes: the scientific Savants, led by the earnest Deedrix; and the religiously fanatical Deons, led by Lexa. The latter worship the Dodecahedron, a mysterious twelve-sided crystal which they see as a gift from the god Ti. The Savants, however, have utilised its power as an energy source for their entire civilisation. The planet’s leader, Zastor, mediates between the two factions, whose tensions have grown greater as the energy source has begun to fluctuate. When Zastor’s old friend the Fourth Doctor gets in touch, the weary leader invites him back to Tigella to investigate and help. When the Doctor, Romana and K-9 try to land the TARDIS on Tigella someone intervenes, trapping them in a time bubble known as a chronic hysteresis, causing them to repeat their words and actions over and over again.
The culprit is Meglos, the last Zolfa-Thuran, a cactus creature who has remained hidden below the surface of his planet in a secret structure. He has summoned a band of ramshackle space pirates called Gaztaks to help him in an audacious plan, and their leaders Grugger and Brotodac are greedy enough to try. Meglos wants to steal the Dodecahedron back from Tigella, as it is a Zolfa-Thuran energy source of immense power. To aid him, Meglos uses an Earthling captured for him by the Gaztaks to occupy and take on humanoid form: and the humanoid form he chooses is the Doctor, whom he has trapped in the bubble. While the hysteresis persists Meglos gets the Gaztaks to take him to Tigella, and infiltrates the city in his new identity. Zastor greets “the Doctor” warmly as an old friend, asking him to examine the Dodecahedron, but others are less sure, especially Lexa.
The Doctor and Romana break out of the bubble by throwing it out of phase, and then land the TARDIS on Tigella – but in the middle of the hostile jungle rather than near the city. As the Doctor heads off to find Zastor, Romana stumbles across the dangerous vegetation – deadly bell plants – and then the Gaztaks, waiting patiently for Meglos to return to their spaceship. She gives them the slip after a while and heads off to the city