Hydra was aplanet invaded by theVoord, led byTarlak. The planet was almost entirely covered bywater, save for a single landmass:Predora. The planet had large, technologically advanced settlements, and a network ofboats andcargo ships, such as theHopeful.

TheFirst Doctor,Ian Chesterton,Barbara Wright andSusan Foreman helped the nativeHydrans defeat the Voord invaders. (AUDIO:Domain of the Voord)