Middletown in 1889 is a small mining village -- but the local mine is about to be closed down. In an attempt to avoid this catastrophe, the owner, Lord Urton built a huge dam across the river to reduce flooding in a promising area of the mine, but to no avail.However, little does he realise that the construction of the dam has disturbed something that was lying dormant in the earth. Something that is now stirring into life...Strange events are afoot, with local deaths that appear to be caused by lightning strikes. Except that the soles of the victims' feet are burned black.It's up to the Doctor to find out what mysterious forces have been unleashed. Perhaps the stranger in town, Nepath, who wants to buy the mine from Lord Urton, can shed some light on the matter...