All is not well at the Wenley Moor underground atomic research station there are unaccountable losses of power-output nervous breakdowns amongst the staff and then - a death! UNIT is called in and the Brigadier is soon joined by Doctor Who and Liz Shaw in a tense and exciting adventure with subterranean reptile men - Silurians - and a 40 ft high Tyrannosaurus rex, the biggest, most savage mammal which ever trod the Earth!