The Mystery of Edwin Drood was a novel on which Charles Dickens was working at the time of his death. In Cardiff on Christmas Eve 1869, Dickens told the Ninth Doctor and Rose that it "lacked an ending". However, he was inspired by their adventure with the Gelth. He intended to introduce characters called "the blue elementals" to help him conclude the novel, and hinted that perhaps he might change the title to The Mystery of Edwin Drood and the Blue Elementals. It remained uncompleted due to Dickens' death the following year. (TV: The Unquiet Dead)
Behind the scenes
As in the Doctor Who universe, the real-life Drood was the final effort of Charles Dickens.