In the fall of 2004, an accomplished, gorgeous Scientologist named Nazanin Boniadi was allegedly selected by officials in the organization to be Tom Cruise’s next girlfriend. Her never-before-told story—of the months inseparable from the star (and his watchers), before she fell out of favor—reveals a complex dynamic that also affected Cruise’s relationships with Nicole Kidman, Penélope Cruz, and, now, Katie Holmes. Maureen Orth investigates.