On the planet Tigella, the Savants and the Deons are irrevocably opposed on one crucial issue: the Dodecahedron, mysterious source of all their power. To the Savants it's a miracle of science to be studied and used to benefit all. To the Deons it's a god, and not to be tampered with. When the power source begins to fluctuate, the whole planet is threatened. Zastor, Tigella's leader, welcomes the Doctor and invites him to arbitrate; yet the Time Lord may not be all he seems...