The Wandering Jew and Hérodiade are condemned to wander the earth until the entire Rennepont family has disappeared from the earth. The connection is that the descendants of the sister are also the descendants of Marius de Rennepont, Huguenots persecuted under Louis XIV by the Jesuits. (Sue never explains how a Huguenot family came to be descended from an immortal Jewish woman who never married or had children. ) The brother and sister are compelled to protect this very family from all harm. After this first introduction, the two appear only very rarely.
