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.

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.