Doctor Who: Criss-Cross

Mostly set in Bletchley Park, this story sees the Doctor stranded in 1944 by a signal jamming the TARDIS's functions. Although alien involvement is apparent almost from the beginning, much of the first half is more like a spy thriller, with both British and German intelligence officers playing a part along with a character who may or may not be a double-agent.

The monster, once it turns up is an unusual one, although not entirely unique. Suffice to say that is well suited to a story rooted in cryptography and secret transmissions, even if, at times, it's a little difficult to follow exactly how it works or what it is doing.

More significantly, perhaps, this is also the first appearance of new audio companion Constance Clarke. (Chronologically, anyway - due to the vagaries of the publication schedule, she had previously appeared in a special release that came out a few weeks before this did). Like some other newly created companions for Six, she is a particularly strong personality, contrasting with the TV version of Peri without being as irritating as TV-Mel. It's an effective choice, blunting some of Six's harshness although, at least to begin with, she doesn't have quite the same charm as Evelyn. I'm interested to see where they are going with her, especially since she is married, and there's clearly some longer-term plot being laid out with respect to her absent husband.

The historical aspects are also good, although all the characters present are fictional. (Thus, among other things, avoiding this contradicting the 8th Doctor novel The Turing Test). It manages to feel like a WWII action story while emphasising brains over brawn, and shows both the limitations and the toughness of those on both sides of the conflict.