"Domingo," the burro the Hollisters were given for a pet the last time they were out West, is the innocent means by which the Hollisters once again find themselves following the mystery trail on horseback.

Because of a near accident, the Hollister children meet and befriend the Blairs, who are from Nevada. They learn that Mr. Blair is trying to sell some of his property on the Tumbling K Ranch, but strange and eerie events have discouraged prospective buyers. Naturally, the minute the Hollisters hear mystery is involved they are eager to be in the thick of it. Their interest is further aroused when a prowler is discovered spying on them in Shoreham. What excitement there is then when they are invited to visit the Blairs and help them solve this baffling problem.

Who is the strange and evil-looking man who tries to cause trouble for the Hollisters on their trip to Nevada? And who is the tall, lanky cowboy, Dakota Dawson, whose actions are so suspicious? And are the strange lights seen on the mountain man-made or some odd trick of nature?

The Happy Hollisters find fun, adventure, danger, and the answers to the many questions posed by the mystery surrounding the Tumbling K Ranch.

Paperback; 195 pages with 20 illustrations and over 20 bonus pages