Why the "Brothers and Sisters" star is suing his ex-employee.

ROB LOWE and his wife are suing their former nanny for $1,000,000.

The couple claims their children's former caretaker, LAURA BOYCE -- who worked for them for most of 2007 -- "betrayed their trust and engaged in a scheme to hurt the Lowes by spreading malicious lies," according to court papers filed Monday in Los Angeles Superior Court. They are suing her for violating a confidentiality agreement, defamation and infliction of emotional distress on their family.

The court papers make no specific reference to what sort of secrets they allege she passed along or to whom she may have told.

He has also filed suit against another former nanny and a former chef, claiming improper conduct.