A trail of cryptic codes sends Ben and Sam deep underground to discover a treasure more valuable than gold.

Ben Coolahan loves his life in Boston in 1873, but when his father passes suddenly, he's sent out west to the family's mine. Ben's father left him only a medallion, coded messages, and a warning to keep them secret. His new guardian, Delilah, an overbearing woman, sends him to work in the mine. Ben finds refuge with Jack, a Cornish miner who shares his superstitions and tales of the knocking. A quick-witted girl, Sam, helps Ben unravel the codes and discover their hidden messages, which lead them into secret chambers and down underground rivers to discover a precious treasure. Faced with protecting it while thwarting Delilah's foreman, they find refuge deep underground. The miners are blasting their way toward the wall of gold. But gold is not what Ben and Sam are protecting, and they'll have to risk the mine's fortune in order to save lives.