A group of palaeontologists have been savagely attacked while carrying out a study of a fossilised cave system on twenty-fifth-century Earth. A party of troopers and Professor Kyle, the only survivor of the attack, are investigating the death of her colleagues when they discover the Fifth Doctor and his companions at the site of the massacre. The time-travellers are immediately suspected. In trying to establish their innocence and find out who-or what-was responsible for the killings, the Doctor is confronted by an old enemy...