On Tigella, two opposing factions are irrevocably divided over one fundamental issue: the Dodecahedron, a mysterious artifact which provides the entire planet's energy. With the Savants and the Deons locked in a crippling stalemate, and their civilization on the brink of collapse, the Tigellan leader Zastor seeks the Doctor's help. But the Doctor and Romana have been trapped aboard the TARDIS in a time loop by Meglos.