The inhabitants of Tigella in the Prion planetary system have a problem: the giant dodecahedron which has powered their underground city since it fell to their planet is now failing. The surface of Tigella is covered with lush, aggressive vegetation, and the Tigellans believe that unless they can find a solution to the power loss they are doomed. The race is split into two factions: the Savants, led by Deedrix, are scientists who believe that the dodecahedron is an artifact which was manufactured, and the Deons, led by Lexa, are religious worshippers who believe it to be a gift from their god Ti.

Zastor, the impartial leader of Tigella, summons the Doctor to come and help, but on its way to the planet the TARDIS is caught in a chronic hysteresis - a loop in time.

The TARDIS has been trapped by Meglos, the last surviving Zolfa-Thuran, whose race built the dodecahedron. Meglos, whose true form is that of a xerophyte, has summoned help in the form of an unruly band of Gaztak mercenaries, led by General Grugger. They kidnap a human and Meglos takes over his body, then transforms it to look like the Doctor.

Meglos arrives on Tigella ahead of the trapped Doctor and tricks the Tigellans into allowing him access to the dodecahedron. When he is alone with it, he miniaturises and absconds with it.

When the Doctor arrives, having escaped the time loop, he is of course accused of the crime. However he escapes and follows Meglos back to Zolfa-Thura where the cactus creature plans to unleash the power of the dodecahedron to destroy Tigella. The Doctor turns the tables and mis-sets the computers controlling the device, thus causing an explosion which destroys Meglos.

The Tigellans are forced to begin reclaiming their planet's surface from the vegetation and the Doctor takes home the human whose body Meglos had stolen.

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