The Prion star system is home to two planets, one of them dead and the other slowly dying. Unbeknownst to the inhabitants of the dying planet of Tigella, their fate and that of the planet Zolfa-Thura are more closely linked than they know.

This week, Paddy and Tricia follow The Doctor and Romana as they answer a summons to help settle a dispute that could save or damn Tigella. Can they make the two factions co-exist or will they fall prey to the schemes of the sinister Meglos?