 Although the series depicts a variety of crimes and perpetrators , Holmes ' conflict with archnemesis Jim Moriarty ( Andrew Scott ) is a recurring feature . Molly Hooper ( Louise Brealey ) , a pathologist at St. Bart 's Hospital , occasionally assists Holmes in his cases . Other recurring roles include Una Stubbs as Mrs Hudson , Holmes and Watson 's landlady , and series co @-@ creator Mark Gatiss as Holmes ' elder brother Mycroft . 
