M00NEY
Adopted children can't always have known they were adopted. Their parents must break the news to them after the child has discovered a secret from their past (an old photograph or a locket, perhaps?) and the news must send the child on a search for his/her biological parents. The character must never consider who raised him/her. He/she must always feel betrayed.
This sort of happens in a story I'm writing...
eek -the search for parents, - they feeling of betrayal, -the locket or whatever. In fact, because she was "posing" as a princess, the whole thing causes an international incident and such. Essentially, according to the "walls have ears" theory, if the Queen would have told her own daughter, the whole thing would have exploded sooner. But that may have been better, now that they've gotten the country that she was betrothed to involved.