One of the least important ramifications of John F. Kennedy's assassination is that it remains forever linked with the beginnings of Doctor Who, if only in the minds of fans. In Matrix, this event cleverly indicates something awry with history; Kennedy - president of fifty-one states - is killed on the wrong date, in a London yet to recover from hysteria induced by Jack the Ripper.

Where the previous book by these London-obsessed authors was a pastiche of several contemporaneous Doctor Who stories with occasional detective story veneer, Matrix is fan fiction styled as homage to the horror story. Despite vague early threats of malign gestalt psychic entities, the nature of the problem faced by the Doctor is eventually clarified, following a story which is in essence a nightmare for both Ace and the Doctor - stranded in a history which never happened, the Doctor abandons Ace to face a mystery from his past, shedding his identity in the process.

Alternative history stories are no longer intrinsically surprising, and JFK and Jack the Ripper have inspired so much writing one might imagine there's little left to be done with them. The dearth of originality in Matrix is compounded by its status as sequel, although the result here is a logical development of the subtler method of linking seemingly discrete stories developed during the show's last couple of seasons. It's tempting to speculate that the writers might originally have found inspiration in Andrew Cartmel's intention to set some of season 27 in the 1960s, although some aspects of Matrix would never have fitted his view of the series - which is not necessarily a criticism.

The text is often unnecessarily ornate and frequently exhibits the portentousness of the McCoy era - particularly in the annoyingly cryptic lines given to the Doctor. While the flavour of fan fiction lingers through appearances by earlier Doctors and other familiar characters, the bleakness of the depiction of 19th century London and the unpredictable twists of plot provide more depth. It might have been more satisfying if all the implications of the Doctor's uncharacteristic and sometimes violent behaviour had been fully addressed, but Matrix remains reasonably successful. One wonders where these authors' very own season 27 will go next.