Some books, such as John Le Carré's Smiley's People, have been successfully dramatised by simply lifting the entire dialogue word for word and sharing it out among the actors. This would never have worked with the Tripods trilogy, not least because Christopher's original characters live in a children's book, and have a tendency to speak with implausible correctness -- never using "you" where "one" is better -- and with only children's book vocabulary. Although a few set-piece speeches survive in the television version, and some of the more poetic paragraphs of narration are transferred to dialogue, everything else had to be rewritten. This alone would make Alick Rowe more of a co-author than an adapter, even without the four episodes' worth which are almost entirely his own (1.9 to 1.12). Five and a half hours is a dangerously generous amount of air-time for a book of about 65,000 words.

Alick Rowe is a playwright with some experience of handling juvenile characters, and whose other TV credits include the adaptation of P. D. James's fairly cerebral detective novel A Taste for Death and Muriel Spark's The Prime of Miss Jean Brodie, the life story of an Edinburgh schoolmistress. Rowe does seem fond of policemen and teachers: his invented Madame Vichot, who comes from Edinburgh, has more than a touch of Miss Jean Brodie about her, and numerous Black Guards are paternal to our heroes. For that matter, so are the Count, Ozymandias, the Schoolteacher and ultimately Julius. To Rowe, the story is fundamentally about growing up and about choosing between two different ideologies, pushed by two different kinds of teacher.

At almost every point, Rowe's ear for dialogue shows. The long interrogation sequence, in 1.13, is a tour de force of screen-writing, but so are more comfortable moments in the vineyard's grape-pressing barn, treading a careful line around desire. Rowe said of himself that he has

a fascination for people and the odd things they say and do... Blame for this can be squarely laid at the feet of my extraordinary and wonderful family whose Border Welsh sense of drama, exaggeration, danger and fun remain with me...

The Vichot family are such a clan, and several other major characters -- such as Daniel and the Duc de Sarlat -- are entirely invented by Rowe. The idea of the Black Guards is also his. A sort of police force on the European continent, they track down "evaders" from Capping. This is in contrast to Christopher's trilogy, where the Tripods are quite unconcerned about the few who evade, since they never cause serious trouble. Rowe's Black Guards externalise the drama, providing a substitute enemy with which the actors can share scenes. Their presence shows the Tripods to be at war with the free men, and Europe to be under a sort of fascist occupying power. At some points, The Tripods is more than a little like Secret Army, the late-1970s BBC drama in which the Nazi occupying forces of Belgium tracked down "evaders" sometimes helped by local farmers: the evaders being English refugees, airmen who had parachuted out of shot-down planes.

Given that television cannot summarise, only show, Rowe is obliged to be more exact about characters only sketched in Christopher's book. Two cases in point are Captain Curtis, who was supposed to be a reliable contact, and the Count, a dull if brave man in the shadow of his wife. It's interesting that in both cases Rowe abandons the original character note in favour of ambiguity and independence: Curtis becomes a mercenary; the Count is a law unto himself who obeys the Tripods but, uniquely, not the Black Guard.

The books are narrated in the first person, and Rowe conveyed this in his scripts, not by having a voice-over but simply by writing the character of Will into every scene longer than about three exchanges, a heavy burden to place on any actor's shoulders: as John Shackley noticed immediately on being given the scripts, he is almost never off camera. Yet the central drama is not Will's story but the interplay between the three travellers, whose trust gradually develops through inconsequential conversations scattered throughout. Henry cuts himself and then is annoyed when the others make a fuss. At another point, Beanpole shows the Parkers a railway line:

WILL: There are metal lines like this near Wherton.

HENRY: Where?

WILL: Behind Stone Farm.

Beanpole knows no more about Stone Farm than we do, but that last exchange reminds us that the Parkers come from the same village, until recently their whole world. It's a small screen moment, dramatising a passing thought in the book:

There was a raised bank, not far from the village, which ran straight for miles, and it occurred to me that Schmand-Fair [i.e. chemin de fer, railway] could be built along that. Or perhaps had been built, long ago, before the Tripods?

The down side of relying on this triangular relationship is that, when the three regulars are divided, the script loses much of its coherence. In 1.2, the scrappiest episode, several neatly directed scenes and some beautiful filming of the sea are held back by unduly long two-handed conversations between Will and Henry, whose opinions are not really different enough to sustain much conversation without a third person present. In 1.7, after Henry and Beanpole have abandoned the Chateau, there is very little dialogue at all, and it's hard not to feel that the Tournament scenes -- archery, jousting, fencing and so on -- drag on, however spectacular.

So much for the characters. The science fiction content of The White Mountains is low, other than the hypothesis itself. The heroes encounter actual Tripods only five times, and the same almost goes for the television version, except that the producers -- not Rowe -- added a couple of cameo appearances on the skyline. (The wordless scene opening episode 1.10, in which a Tripod heaves itself above the vineyard, was written in by the director, Christopher Barry.) Special effects such as Tripod movements were not cheap, but Rowe could probably have found other science fiction motifs if he had wanted: he did not, preferring to stretch out the narrative and adhere to one view at least of the mood of the book. Almost every scene has some textual justification. His two large-scale extensions of the story were both expanded from paragraphs. Here is the entire original of the Vichot vineyard sequence:

Towards evening, we climbed up through fields closely set with plants, supported by sticks, on which were clusters of small green fruit. These would be picked when they were fully grown and ripe, and their juice squeezed out of them to make wine. There had been a few fields of them in the neighbourhood of the castle, but I was amazed by how many of them there were here, and how the fields -- or terraces, rather -- were laid out to catch the rain and sun. I was hungry enough to try one or two of the larger fruits, but they were hard and sour, and I had to spit them out.

It isn't easy to squeeze fifty minutes of drama from that: what Rowe does is to drop in a self-contained play about the lives of the vineyard family, using it as a chance to bring more female characters in. (Being a book for boys set in a male-dominated society and written in the 1960s, The White Mountains is pretty thin on girls.) On the previous page of the book a village fete is mentioned, with horses and traps decorated with ribbons: that, too, had to fill out to a whole episode. But then the book is built around travel, and this immediately raises the problem of conveying a sense of time and place moving on. Here's another typical paragraph from the book:

We made good progress through bright moonlight and, when dawn came, were well clear of familiar country. I called a brief halt, and we rested, and ate half of one of the loaves with cheese, and drank water from a stream. Then we continued, more and more tired as the day wore on and the sun scorched its way up through a dry blue sky.

Everything here is troublesome on-screen: night and day, many locations needed, nothing actually happening, no dialogue, the difficulty of finding a stream people might plausibly drink from, how to convey tiredness and so on. Rowe's script requires an enormous number of locations, but even so it sometimes struggles to give the sense of a long journey: a casual viewer of episode 1.8 would not realise that it's supposed to be taking place over an entire week. The dateline titles are helpful here, as July gives way to August, September, October and at last November. The script reminds us periodically that the travellers are up against the season, needing to reach their destination before the worst of the winter.

That we are in France, not England, is a point insistently made by both script and design. Just about every French national icon appears somewhere, from a Parisian road-sign Rue Jeanne d'Arc to portraits of Charlemagne and of Napoleon. Calais, the Eiffel Tower, the Arc de Triomphe, wine-pressing, chateaux, the accordion, a tumbril, French folk-songs (sung in French), church festivals and the French legal system all appear on screen. It is a France of regional government; in the Jura episodes, for instance, we are told that the centre of authority is Dijon, once the seat of the medieval duchy of Burgundy. There is talk of "le loi de la région" (region, not country), and Rowe dramatises the traditional love-hate relationship of provincial France with Paris in a revealing argument. Eloise has just introduced her new fiancé to Saclay, her father's acerbic former tutor:

ELOISE: The Count comes from near Paris, Will.

WILL: It must have been a wonderful city.

SACLAY: You are entirely wrong. History teaches us it was over-populated, disease-ridden and a centre of evil. It dictated the ways of an entire nation. Among the many benefits of the coming of the Tripods the destruction of Paris, and all such cities, is to be greatly applauded.

Note that the word "Paris" does not occur at any point in the books, where it is a forgotten ruin. On camera there are far more place-names: partly because conversations happen live (not in reported speech), so that proper nouns are hard to duck. John Christopher's intended geography is more or less closely followed. A tiny moment in episode 1.2 has Ozymandias spreading out maps of England and France: though these exquisite props are only very briefly in shot, a good deal of care has been taken over these few seconds. The camera shows Ozymandias pointing to "Rimney" at a place corresponding to the stretch of sandy coast between Rye and Romney (note that Rye is where John Christopher was living in 1984, and where some scenes were filmed). Moments later, the French sheet comes into view, on which a network of orange routes seem to represent routes used by the free men. As shown on this map, the Orion sails from Rye to the nearest spur of France, landing at Calais (diverted, as it happens, from Boulogne): this information is confirmed in French, briefly and ten whole episodes later, in the long interrogation sequence.

But the scripts do deliberately deviate from the geography of the books here and there, when there's good reason to. It's a dramatic convenience for the travellers to have a sense of being, at one point, near the City of the Tripods: indeed, they briefly sight it at a distance. This is useful both in concentrating the viewers' minds on the Tripods and the future destination of the plot, and also in provoking Will to talk about Eloise's fate, since she has been taken to the City whereas Henry and Beanpole do not yet know this. Geographically, though, there's no sensible way it could happen (given what we know of the City's location in the second series), and it doesn't happen in the books.

Rowe's most deliberate geographical change is well signalled: when the boys speak of the White Mountains,

MADAME VICHOT: D'you perhaps mean the Alps? For they are, indeed, white mountains...

The Alps indeed, but Rowe moves the refuge of the free men, the Tunnel: he places it at Mont Blanc -- the White Mountain, that is, and the highest peak in Europe, by definition the corner-point between France, Switzerland and Italy. This is more dramatic, and gives the travellers an unambiguous destination -- for, as the script remarks, the Alps occupy five countries, so that it makes no sense to be heading simply for "the White Mountains". This point is glossed over in the novel, where it is assumed that a map gives all necessary instructions.

Very unusually for television, the first series of The Tripods has no "translation convention": the French characters actually speak elementary French on-screen, which our heroes sometimes understand and sometimes don't. ("Ils n'ont pas les calottes!": calotte means a priest's skull-cap, thus the Cap; do not confuse with culottes, underpants.) Except for one brief scene in which Beanpole and Helen, both native French, choose to seduce each other in English -- perhaps for the intellectual exercise -- Rowe does quite well in finding reasons why various French people, met along the way, speak enough English to make sense to the viewer. The effect lends the scripts more than a little charm, but above all, it roots them in the land they are travelling through. The White Mountains is a book founded on a romantic vision of Europe, and Rowe's most successful contribution is to convey that by locating it in rural France.