A spin-off from the established BBC medical drama Casualty set in the same hospital, in the fictional city of Holby. The show follows the lives of medical and ancillary staff at the fictional Holby City Hospital. It began with eleven main characters in its first series, all of whom have since left the show. New main characters have been both written in and out of the series since, with a core of fifteen main actors employed on the serial at any given time.