Incredible Story. The creature apparently has a natural origin, like the hound of the Baskervilles, only nobody knows what it might be. The narrator himself is guessing, and we are left guessing too.
Dr James Hardcastle, who is convalescing in a Derbyshire farm, discovers Blue John Gap, a Roman Derbyshire Blue John mine. He begins to investigate the mine and the extensive underground formations to which it connects, despite the warnings of a local farmer who claims it contains a monster that has been stealing his sheep.
Hardcastle hears a large creature moving in the cave, and uncovers more evidence that it has been preying on local sheep. He determines to face it alone, as the colleagues whom he asked for help dismissed his claims as madness. He manages to wound the creature, and is bady hurt himself doing so.
The local people, who believe Hardcastle, seal the mine after the creature has returned to it. Hardcastle dies from trauma, leaving a description of his experience to try to convince his London colleagues.