Tracking down the German Shark

Robert Harris has followed his best-selling novel, Fatherland, with a code-crackingly brilliant new thriller. By Penelope Fitzgerald; Enigma by Robert Harris Hutchinson, pounds 15.99

In Robert Harris's brilliant alternative-history thriller, Fatherland, Hitler makes peace with Britain, without much difficulty, in 1944. Discovering, at long last, that British cryptanalysts are reading signals from the Enigma machine, he recalls the U-boats to their Atlantic bases and changes their cipher. "Picking off merchant shipping had been easy after that. England was starved into submission." In Enigma, Harris has second thoughts.

This new story is played out among the codebreakers of wartime Bletchley Park. The Park had been staffed at the beginning by a few problem-solving dons - Bletchley was chosen because it was an equal distance from Oxford and Cambridge - accommodated in damp wooden huts. By 1943, Harris's starting point, it had grown into a prefabricated camp for seven thousand people working shifts and dealing with three thousand intercepts a day.

Enigma sets out with an energy and fast forward movement which it never loses. Tom Jericho is a quiet young Cambridge mathematician, a pupil of Alan Turing, who persuades him down to Bletchley. Tom finds himself in Hut 8, the Naval Section, working on Shark, the operational cipher used by the U-boats. When the Germans add a fourth enciphering rotor to the U-boats' version of Enigma, Hut 8 is in check. Bletchley's primitive electro-mechanical computers can't sort out the added load of permutations. But after three weeks, when nerves are worn ragged and Tom's brain is "beyond tiredness", a way into the problem lights up like a sudden new landscape before him.

Whether his solution is striking enough to precipitate his subsequent nervous breakdown, I'm not quite sure, but Harris is obliged in this novel to simplify the unsimplifiable. Tom, in any case, is sent on sick leave. Almost before he's recovered, though, he is recalled to Hut 8 where Shark has got away from them again. The German High Command has changed its weather code, which provided a constant and has so far been Bletchley's permanent key to their naval Enigma Variations.

At this point, as the sick and exhausted Tom gets down to work, Harris powerfully complicates his plot. Tom is obsessed with Claire, a high-born but dodgy filing clerk who has stolen (but why?) a series of old intercepts. Meantime, the Battle of the Atlantic is in the balance, and American observers have arrived at Bletchley, demanding control, now that the US is at war, over the whole cipher-breaking process. Tom, it's true, has been lightning- struck by another idea, that there might be a way back into Shark through the contact signals of a massive concentration of U-boats, known to be advancing on a North Atlantic convoy. But to get anything useful means waiting until the U-boats have got right in among a million tons of Allied merchant shipping - bloody slaughter, and who's to judge whether it will be "worth it in the end"?

Robert Harris is a superb writer of thrillers because he does, and doesn't, accept their conventions. In Enigma, the head of the Naval Section, who has no faith in Tom, comes to grief. Vicars are sanctimonious, college porters are comedy figures, policemen have brick-red faces. One of Tom's girls is chilly and buttoned-up, but will certainly relax in time. The other, Claire, the intoxicating blonde, disappears, and is doubly, or perhaps triply, not what she seems. Thriller-readers will be at home with all these things. But Harris transcends them. He expertly recreates the past with a sense of admiration, anger and pity, from the unswallowable wartime food to the sounds a U-boat registers when a convoy passes overhead. What's more, he understands the patient boredom of Bletchley's massed clerks, and, seemingly quite apart from them, the not-quite-sane beauty of the specialist intellect at work.

Harris makes one or two references to my uncle, "old" Dilly Knox, who wasn't quite 60 when he died in 1943 after discovering the first significant "way in" to Enigma. Dilly would probably not have flourished in a computerised world, and neither would Tom Jericho. He finishes (as most of Bletchley's academics dreamed of doing) back in his old college, "moving off purposefully to his staircase, passing out of sight".