At the end of the 19th century, Silas Shacklady was obsessed with his mine in the Snowdonia mountains. Digging ever deeper in search of a Motherload, Silas disturbs dark secrets and powers hidden for centuries within that subterranean world. In 1897 the mine was sealed after a cave-in buried Silas and over two hundred men, caught forever within its black maw.

The mystery of how this happened and the strange presence of The Lady Emily, an ancient steam engine, draws Nick, Wendy and Alan into a dangerous underground adventure. One which finds them trapped in a battle between malevolent forces, risking all their lives and creating profound consequences for many.