When she joins Aunt Japonica and Aunt Thessalonika on a trip to America, nine-year-old Harriet meets yet another relative--Aunt Formica, a cowgirl who is having trouble with some clever and mysterious cattle rustlers.
When her absent-minded inventor father suddenly remembers that he has five sisters, nine-year-old Harriet Bean, who has never heard of them before, determines to find her unknown aunts so that the unfinished family portrait can be completed.