The lunatics are taking over.

Every few years, someone has a go at a Doctor Who Murder Mystery; this tradition can be traced at least as far back as the 1977 TV adventure The Robots of Death, a template rarely (if ever) bettered. Peter Darvill-Evans' second BBC novel presents itself as another addition to this genre - but as with this author's previous Independence Day, early momentum is allowed to dissipate.

Given that Asylum is a slow-moving tale, this inevitably detracts from the book's effectiveness - which is a shame, because a lot of it works. Incorporating an older Nyssa meeting a younger Fourth Doctor could so easily be a silly, meaningless gimmick - but Darvill-Evans pulls it off. The medieval Oxford setting for the majority of the book is also grimly convincing, although the lengthy afterword in which the author describes his research errs on the side of providing rather too much information. After arriving in the 13th century, though, the Doctor and Nyssa retreat into the background, the latter in particular only emerging from hiding to re-emphasise the ennui and disillusion which have crushed her spirit. This needn't be a problem - that it is here in Asylum is due partly to slenderness of plot (certainly compared with the rich textures of character and theme) and partly to the rather po-faced method of presentation; 'gritty' and 'humourless' are difficult targets to hit in Doctor Who.

One very successful subtext is that everyone has secrets and a hidden agenda - Asylum ultimately becomes almost as much a thriller of primitive political intrigue as the time paradox SF tale or medieval whodunnit suggested by the early chapters. That such aspects are strong while the novel remains thin in terms of plot and lacking in the substance often provided by length imply this might have worked better as a short story or novella. There's not really enough going on here to sustain even the 220 pages we are given, but nevertheless it's diverting enough if you're willing to cut it the necessary slack. Just don't expect a whimsical, pacey adventure...