(Back cover) When it's discovered that quartz gold is being "highgraded" (stolen) from a hardrock mine in California's historic Mother Lode, Hildy Corrigan tries to help the mine owner solve its baffling mystery. All they know is that gold is being stolen. Who is doing it and how they are smuggling the gold from below ground is hidden in the mine's darkness.
Hildy enlists the help of her cousin Ruby and friend Spud to gather information and follow clues that thrust them into unexpected jeopardy from unknown adversaries. Despite Hildy and Ruby's worst fears, when the clues point them into the depths of the underground mine, they must follow. Racing the clock to save an injured boy, the three friens make their descent into the greatest danger they have ever faced.