Roots: The foreword contains a brief list of the works of William Hope Hodgson, creator of Carnacki the Ghost-finder, including his nine Carnacki stories. One of these, The Whistling Room, is reprinted at the end of the novella (though not in the paperback edition). There are references to Tom Sawyer and Huckleberry Finn, Sherlock Holmes, the Marquis de Sade, and Macbeth.
Continuity: The Doctor likes champagne cognac. He carries a policeman's whistle. He knows of Carnacki by reputation, having read accounts of The House Among the Laurels, The Whistling Room, and The Haunting of the Jarvee.
One of the walls of the TARDIS control room converts into a single massive scanner screen that the Doctor seldom uses. The Doctor is able to program the TARDIS to follow a teleportation wave by plugging in a plug board. He keeps spare wires and cables for the TARDIS in a chipped teapot.
Links: Zoe refers to the Wheel (The Wheel in Space).
For Carnacki, the events described here take place before The House Among the Laurels, The Whistling Room, and The Haunting of the Jarvee.
Location: China, December 1800AD, and Kent, December 1900AD.
Unrecorded Adventures: The Doctor may have met Daniel Dunglas Home.
The Bottom Line: A splendid period piece, boasting great characterisation both of the regulars, Carnacki, and the supporting characters, and successfully incorporating magic into Doctor Who without the use of technobabble. Cartmel succeeds in giving Carnacki and the Doctor a mutually beneficial working relationship, neither overshadowing the other. And the prologue is genuinely chilling.