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 doctor gets in touch, the weary leader invites him back to tigella to investigate and help. when the fourth doctor, romana and k9 try to land the tardis on tigella someone intervenes, trapping them in a time bubble, causing them to repeat a small "pocket of time" over and over again.