A visit to the bustling city of Sacramento should be the trip of a lifetime for twelve-year-old Jem Coulter. He's never traveled so far from Goldtown. But something's not right. Pa won't say what, but Jem can feel it with every bump of the stagecoach. And then, nearing the end of the tiring, two-day journey, the stage is held up. Highwaymen calling themselves the Knights of the Golden Circle haul away a precious shipment of gold.
So, why isn't Sheriff Coulter upset over the robbery? Or by the bandits' claim that the holdup is within their rights? The answer turns Jem's carefree city holiday upside down. While exploring the river wharf and enjoying an impromptu tour of a paddlewheel steamboat—courtesy of a new friend—Jem, Nathan, and Ellie stumble across a dangerous smelling operation. The the bandits show up, and Ellie disappears.
With Pa off on business and Aunt Rose worried sick, Jem must use all his wits to find and rescue his sister before the side-wheeler steams downriver and out of reach.