Rent Doctor Who: Meglos (1980)

On Tigella, two opposing factions are irrevocably divided over one fundamental issue: the Dodecahedron, a mysterious artefact which provides the entire planet's energy. With the Savants and the Deons locked in a crippling stalemate, and their civilisation on the brink of collapse, the Tigellan leader Zastor (Edward Underdown) seeks the Doctor's help. But the Doctor (Tom Baker) and Romana (Lalla Ward) have been trapped aboard the Tardis in a timeloop by Meglos, the last of the Zolpha Thurans, who will stop at nothing to steal back the awesome power of the Dodecahedron...