The year is 1938—two years since Sophie, Molly, Mark, Harriet, Leticia, and Posy made international headlines when they uncover the buried treasure of the notorious Brother XII. Since then, life has been decidedly un-exciting for the adventure-loving crew, who feel more at home on the rolling deck of a sailboat than in their stuffy boarding school. But once again, Uncle Bert (a.k.a. Captain Gunn) comes to the rescue when he invites the whole gang on another holiday abroad.

The kids trade in their pirate capsfor cowboy hats and head off on a cattle drive in Interior BC. But when they run into a familiar villain, the trip takes an ominous turn culminating in a dangerous mission to an old mine. Will their adventurous ways get the better of the kids this time, or will fortune smile upon them yet again?