A boy and his loyal dog respond to a mysterious message and, with a friend, travel west to find a missing uncle. Their episodic journey features telegrams, sudden pursuits, a lonely cabin, and a wild ride that leads to a dry mine and hidden caverns. Strange noises and a secret camp prompt scouting and clever problem-solving; the boy makes a telling drawing and Ruddy’s instincts prove crucial. Together they uncover the plot that diverted Lost River, rescue the uncle, restore the watercourse, and recover the mine, with the plot emphasizing teamwork, resourcefulness, and courage in a series of small adventures.