Imagine how shocked you would be, if like ten-year-old Colin Mason, you were the oldest (smartest, best) kid in a family of four, and then you found out that all the time you had a secret older sister, or so strange Mrs Newby, the neighbor, claims- "an awful, awful, bad, bad, girl-Loretta Mason Potts." Who? What? Wait!... But this is only the first of many surprises that lie in store for Colin, as things get curiouser and curiouser very fast. Loretta (a glum gangly girl and with what a mouth!) comes home and before you know it, Colin is secretly following her down a a secret tunnel leading from the closet to an astonishing castle where a charming and beautiful countess keeps court attended by a dapper and ever-obliging general, and they just love Loretta to death (especially when she's rude), si much that they want her to stay with them forever. What is the secret behind this strange other world and how does it connect to the many many secrets in the Mason family? (How, for example, did Mother's ermine coat come to be lying under Mrs Newby's hedge, all stained with plum juice and peanut butter?) It'll take a spellbinding, hair-raising adventure for Colin and Loretta and Mother and the children (and don't forget Mrs Newby) to work that out.