Out on a leisurely shopping trip, Nancy, Bess and George encounter an odd French-Chinese perfume saleswoman, who is reluctant to sell a particular fragrance to Bess. On a return train trip to River Heights, they ponder her odd behavior and encounter the malnourished Millie Burd, who is on her way to "the city" to seek employment. Millie becomes acquainted with the girls, and when Nancy accompanies her to a job interview, she uncovers a mysterious code in the office.Millie's grandmother, owner of Red Gate Farm, welcomes the trio and Millie to her farm where they plan to vacation as paid boarders, and assist with waiting on other boarders. Once on the farm, Nancy uncovers a strange group called The Black Snake Colony - a "nature cult" - who are tenants on an outlying portion of Mrs. Burd's f