Detective Murdoch investigates a series of robberies where the thieves have dug through the the ground and into various shops or banks. The thieves only steal diamonds however. Murdoch suspects the thief is using some type of boring device and that he and Julia may have actually felt it burrowing under the ground the night of the first robbery. Constable Crabtree develops his own theories after interviewing Elva Gordon, a deep cave explorer who believes the Earth is hollow and that there might be an entire world of people living underground. Crabtree now believes "mole people" may be at work. That theory doesn't impress Murdoch who decides to use modern technology to track the boring machine.