While accompanying U.N.I.T and the Brigadier, the Doctor investigates an attempt by Professor Stahlman to drill through the Earth’s crust.  The drill begins to leak a green liquid which turns anyone who touches it into a primaeval creature with a lust for heat.  The Doctor accidentally ends up in a parallel universe whilst experimenting with a partially repaired TARDIS console, and finds the project more advanced than in his universe, and antagonistic alter-egos of his friends.