Mark Gattis has been a writer/actor with "Doctor Who" since the revival series began. He wrote nine episodes, featuring the Ninth, Tenth, Eleventh, and Twelfth Doctors - as well as making a few appearances as an actor. Jessica and Phillip travel through his many episodes, looking for common threads.
In their look into the TARDIS library, the couple revisit the first season "Sherlock" episode, "The Great Game." This episode was written by Gatiss, who also co-developed the series --- as well as starring as Mycroft Holmes in the episode!