Tiermann's World: a planet covered in wintry woods and roamed by sabre-toothed tigers and other savage beasts. The Doctor is here to warn Professor Tiermann, his wife and their son that a terrible danger is on its way. The Tiermanns live in luxury, in a fantastic, futuristic, fully-automated Dreamhome, under an impenetrable force shield. But that won't protect them from the Voracious Craw. A gigantic and extremely hungry alien creature is heading remorselessly towards their home.