For there on the screen will be the Monarch butterfly, a splendid orange and black insect which every year makes a remarkable flight from the Great Lakes of North America to Mexico. And there on the background will be a great lake. Except the lake on the programme will not be Erie on the US-Canada border, but Windermere in Cumbria.
Viewers will be unlikely to notice. Indeed, the Corporation has gone to some lengths to hide the fact that it decided to save expensive airfares by filming the scene in Britain: the film was shot in dim light, at night and in the early morning.
Yet the BBC left tell-tale traces behind it - in the shape of 30 of the Monarchs that were taken to Windermere for filming. They escaped, fluttering across the Lakeland fells and exciting local naturalists. One contacted an ecologist for the Lake District National Park - only to be told that what he had come across was a BBC extra.
Stephen Dunleavy, a researcher on the programme which was produced by the BBC's Natural History Unit in Bristol, stressed that the whole exercise had been authorised. The BBC had received a Department of Environment licence covering the accidental release of animals into the wild.
Every autumn, said Mr Dunleavy, the Monarchs mass in their thousands on Lake Erie, close to Toronto. Last autumn, the film crew went to Canada to shoot the event but it was a terrible year for Monarchs - the worst ever. This year, with the programme scheduled for January, instead of going again, they decided to play safe.
"We were planning to go back, but then we talked about a reconstruction," said Mr Dunleavy. "We needed shots of Monarchs by trees by water and we were in the cutting-room at the time."
The film crew took 100 butterflies, bred specially at the Butterfly House at Syon Park in west London, and released them on the shores of England's largest natural stretch of water. The film of them coming together for the flight to Mexico was duly shot and that was the end of a budget-saving exercise well done.
Unfortunately, despite the best efforts of four staff to catch them all, 30 butterflies flew away. Ever since, naturalists in the Lake District have been marvelling at the sudden appearance of Monarchs, with their four-inch wing-span and distinctive, brilliant markings, in their midst.
A spokeswoman for the BBC Natural History Unit defended the decision to introduce the Monarch to Windermere. She denied that it had been a budget-saving exercise. She said: "What we filmed was real behaviour, of what we know happens." In Canada, perhaps. But not in Cumbria.Reuse content