The books are concerned with adolescent girl detectives— a concept Baum had experimented with earlier, in The Daring Twins (1911) and Phoebe Daring (1912). The Bluebird series began with Mary Louise, originally written as a tribute to Baum’s favorite sister, Mary Louise Baum Brewster. Baum’s publisher, Reilly & Britton, rejected that manuscript, apparently judging the heroine too independent. Baum wrote a new version of the book; the original manuscript is lost. The title character is Mary Louise Burrow.

In this, the fourth book of the series, Mary Louise and friends form a group dedicated to supporting the soldiers in World War I, and she brings Josie O’Gorman in to spoil a treasonous plot against the government. (Summary from Wikipedia and Sibella Denton)