Caught in a temporal trap, the TARDIS materialises in Prague, where the Doctor is immediately captured by a military race of armoured elephant-like aliens known as the Crei. Now a prisoner of the recently-promoted Sergeant Kandula, the Doctor learns that the Crei are searching the city for a valuable component known as the 'Tipping Device', a single atom of the isotope Gold-197 that was lost in a hyperspace collision with a specific section of Earth; the entire section of Prague has since been taken out of the space-time continuum and brought aboard the aliens' gigantic spacecraft, its populace left behind so that the Crei can search unhindered. The Doctor offers to help Kan locate the component, and together they search the city; however, the Time Lord soon surmises that the device is in