In the 1880s, the booming mining town of Rainbow Ridge lives in the shadow of Big Thunder Mountain, where wealthy mine owner Barnabas T. Bullion has built his fortune. When his adventurous daughter Abigail returns home, she crosses paths with Jack Reed, a former railroad engineer hired to investigate a series of mysterious train accidents and stories of locomotives moving without drivers.
Their investigation leads them to Barnabas’s corrupt foreman, G. Willikers, who has secretly discovered a rich vein of gold deep inside the mountain and plans to steal it with the help of hired gunmen. As Jack and Abigail uncover the conspiracy, they are drawn into shootouts, horseback chases and a growing romance, while strange events inside Big Thunder become impossible to explain.
With the help of old miner Sedona Sam, his dog Digger and the unpredictable dynamite-chewing goat Billy, they learn that the mountain has a long history of unexplained disasters whenever men dig too greedily.
Willikers eventually seizes the mine and escapes with several train cars full of gold. Jack, Abigail and Barnabas fight back in a violent battle across the mining camp before pursuing him into Big Thunder itself.
The final chase becomes a wild race through collapsing tunnels, waterfalls, broken bridges and abandoned shafts, while a mysterious driverless locomotive joins the pursuit. Jack boards Willikers’s train, and after a brutal fight, Willikers chooses the gold over his own safety and plunges with most of it into a ravine.
One surviving car of gold is recovered, enough to rebuild Rainbow Ridge and reward the miners. Barnabas finally puts his daughter before his fortune, while Jack and Abigail admit their feelings for each other and leave town together.
As they disappear into the distance, an empty locomotive deep inside Big Thunder Mountain slowly begins to move on its own. And Billy keeps chewing dynamite.
