Therefore, its known narrative elements are not a part of the Doctor Who universe. It may have been the basis for a similar story in another medium, however — and that story may indeed be a part of the DWU.
The original storyline had featured the Sixth Doctor and Mel, and a silver cat that would've been the cat from the Doctor's lapel and was supposed to be the spirit of the TARDIS. Platt had intended to show a darker side to the Sixth Doctor and make him frightening again.
Platt later adapted the story as a novel in the Virgin New Adventures, instead featuring the Seventh Doctor and Ace. Editor Peter Darvill-Evans thought it was such a wonderful title that he decided to use it as an umbrella for a three-colume series, though the trilogy idea was never developed beyond the odd cross-reference before deciding to focus on the silver cat. (DWM 305)