The Doctor and the Demon

"The Doctor and the Demon" is the seventh episode in the eighth season of Doctor Who, set in Victorian London where the Cult of Dagara are sacrificing young women to aid an inter-dimensional entity, Dagarothon, enter this world.

Synopsis
The Doctor, Billy, and Isabella arrive in Victorian London, where the sinister cult of Dagara are kidnapping and sacrificing young girls. The Doctor seems particularly worried about the situation, and the time travellers soon realize the cult are summoning the inter-dimensional entity Dagarothon, whose influence somehow spread into this era via a time hole.

Continuation and reference

 * Many characters encountered throughout the story belief the Doctor to be Sherlock Holmes.
 * Dagarothon may be a Daemon, like Azal in the third doctor story The Dæmons.