After meeting a pseudo-Alan Turing and almost-Enigma Machine in Doctor Judson and the Ultima machine in The Curse of Fenric, it was only a matter of time before Doctor Who ended up at Bletchley Park, the home of the actual Enigma Machine and, although Turing himself doesn’t appear, Criss-Cross is a story steeped in the world of codes and espionage.

Criss-Cross is a great title for this story. It is the code name of a British/German double agent but also reflects the codebreaking, the radio signals and the way the plot criss-crosses back and forth from location to location. It’s an absolute roller-coaster of a story which builds and builds to a thrilling conclusion.

Central to the story’s success is the brand new companion introduced here, Leading Wren, Mrs Constance Clarke. Played with aplomb and distinctly stiff upper lip, by Miranda Raison, she is immediately likeable and the kind of companion who, like Dr Evelyn Smythe, is perfect for this story’s Doctor – Old Sixie. Insisting on being called Mrs Clarke rather than Constance, she brooks no nonsense from the 6th Doctor and yet quickly recognises his brilliance and accepts his disarming charm as the genuine aspect of his persona is. With Maggie Stables having sadly passed on, it’s marvellous to have a new companion for the 6th Doctor who returns to a similar dynamic.

And this is a great story for the 6th Doctor. The story begins with Doctor John Smith already ensconced in an office at Bletchley with his strange blue box in the corner (which had to be carted upstairs by some strapping young men). He is co-opting the machines and Wrens of Bletchley to help him work out why the TARDIS has died.

It’s traced back to radio waves and mysterious golden eggs and effectively, the TARDIS has been scuppered by sand in the engine or leaves on the line – advanced technology being fouled up by something simple and basic.

The golden eggs are at the heart of the mission being carried out by the double-crossing, double agent – Criss-Cross. The Germans, of course, have discovered a selection of the eggs and want to unlock their mysteries to further their war effort. Soon, the Doctor and Mrs Clarke are aboard a German submarine in the company of Agent Criss-Cross and confronting Hugh Fraser’s Nazi scientist, Dr Schwartzman.

The claustrophobia of the submarine (especially when the hull is breached) is effectively conveyed and contrasts well with the grounds, huts and offices of Bletchley Park. There are some properly tense scenes aboard the submarine as this is where the aliens – Vardan-like radio-wave creatures who exist on an entirely different plain – first manifest. The Waveform work well on audio and yet I felt I had a really clear visual image of them. The mark of a good audio is one where the visuals come readily to the imagination and Criss-Cross is a story where everything simply popped into my mind – characters and locations all effectively conveyed to the listener.

There are a number of stand out scenes throughout the story. Mrs Clarke’s first confrontation with the Doctor due to his treatment of her ‘girls’; the first appearance of the Waveform; the death of a lovestruck couple (with heavy echoes of the camper scene from The Stones of Blood; and most striking of all, the resolutions of Doctor Schwartzman’s and Agent Criss-Cross’s story.

Schwartzman ends up at Bletchley and the commanding officer there, Harris, reluctantly allows him to aid the Doctor, Mrs Clarke and another Wren, Sylvia, in constructing code which will disrupt the Waveform. Knowing that he cannot return to Germany with the knowledge of Bletchley’s work, Harris is left with only one option – the choice he makes is shocking and affecting and really packs a punch at the end of the story.

Agent Criss-Cross’s fate, even if it is highly reminiscent of Jafar’s at the end of Disney’s Aladdin, is also terrifying.

I had actually listened to this audio before but remembered little about it. This second listen saw it rapidly shoot up the rankings to become one of my favourite BF releases. It’s a striking setting; a rattling rollercoaster of a plot; strong characters; a superb new companion and a hugely fun performance from Colin as the 6th Doctor. Highly, highly recommended.