Wildlife biologists, Whip Sawtell and Cat Bonner, enter an abandoned mine near Thermopolis, Wyoming to conduct a bat inventory. Along with bats, they find two dead men: one shot more than a hundred years ago, one killed very recently. An explosion at the mine entrance traps the two biologists underground. Before escaping they stumble upon a hidden cave and artifacts that point to the former owner of the mine, the famous outlaw, Butch Cassidy. Sawtell discovers his old foe, a Wyoming state senator who now owns the ranch and mine, may be involved. Is a modern-day band of brigands murdering people to protect a hundred year-old outlaw treasure? Or is this about something even more sinister? Sawtell identifies the killers then tracks them horseback over the Bighorn Mountains on the infamous 1880s-era Outlaw Trail that leads to the Wild Bunch hideout at Hole-in-the-Wall, Wyoming. Eventually the pursuit takes Sawtell to a ranch along the Mexican border near Bisbee, Arizona where he exposes a bizarre international smuggling operation.