Spies of Warsaw: A story of derring-do, set in a city the Nazis were soon to destroy One of the interesting things about Spies is that war is the most important fixture in the drama - and yet, like Godot, never actually appears. The hero of the tale is the one man who does expect it to turn up: Tennant's Jean-Francois Mercier, a (fictional) Warsaw-based French agent who becomes convinced that Hitler is concocting invasion plans. "He has some undercover contacts that he's working on, and he does a bit of crawling under barbed wire, and he starts to uncover things that, actually, his French superiors don't want to know about," says Tennant. The job of the actors, in a sense, was to perform a mental equivalent of what Poles did with the Old Town: forget 1944; construct a pre-existent world. "When we look back on World War Two now, we look back with a sense of a certain inevitability," Tennant, 41, explains. But if you were actually living through that, it's important to remember that people would always have been looking for a way out. And, of course, in Britain and across Europe, appeasement was a hugely popular movement. There was this sense that war might be avoided. That Hitler was a bit of a rogue, but he wasn't all that bad and if we tolerated him a bit we could get him on-side. It's this climate of anxiety that makes 1938 a more dramatically rich setting, in some ways, than the conflict itself - at least according to the men who adapted the script, Porridge creators Dick Clement and Ian La Frenais. "As for World War Two, how many more stories can we really tell about it?" muses La Frenais, 77, when I catch up with them. They are sitting outside a palace near Warsaw; La Frenais, impish, with ageing-rock-god hair, Clement, 75, the softly spoken straight man. What they loved about Spies, says La Frenais, was its inherent tension - "that extraordinary sense of the world being on the brink of cataclysmic change." That and the fact that it's a spy thriller, which, despite a career that spans The Likely Lads, The Commitments and Never Say Never Again, is fresh territory. "Actually we love this genre," says Clement. In fact, for years we wanted to do Le Carré but never managed it. Alan Furst is the nearest thing, in a way. Great plots. Wonderful characters. And that sense of uncertainty surrounding everything; particularly with the romance that you get here, which gives it that quality of Casablanca - of love that's threatened by events. Tenant is keen to point out that (as with Casablanca) the fact that some of those events are real gives Spies its punch. It's looking at things from a historical perspective. And I think that gives it more texture than a wham-bam, derring-do story - though it has all that stuff, too. Spies of Warsaw begins on BBC Four on Wednesday 9 January at 9.00pm