The Betrothal of Sontar

The Tenth Doctor and Rose Tyler land on the icebound world of Serac where they encounter the Sontaran crew of the Betrothal of Sontar, a mining vessel commanded by Colonel Snathe, whose quest for the "Worldbane", a source of incredible power, has driven him insane. As the Doctor attempts to help Snathe find the Worldbane — mainly in order to recover the TARDIS — Rose befriends Lerox, an unusual Sontaran who possesses a conscience, and who holds the key to saving not only the Doctor and Rose, but the homeworld of Sontar itself.