Cauldrons bubbled, paddles stirred, pumps rose and fell. The dye selector scurried along seeking indigo and sunflower to make that special shade of green for Familyfresh.

Malcolm Rust loved machinery and money, in that order. Childhood visits to industrial museums had given him a love of pistons and presses. The only history he was interested in at school was of Victorian valleys filled with furnaces and engineering entrepreneurs making a mint, so they could build great houses on top of hills looking down on their wealth. His weekends as a teenager had been spent scouring the country for redundant factory equipment and thinking of money making projects to fund his hobby.

He had no interest in the environment, except as the provider of water courses to power mills, until he met Melissa. She worked with his mother at the new Veganarium that had replaced the cheese and bacon shop. His mother needed a job, but for Melisa it was her whole way of life.

As far as Malcolm was concerned food was fuel, the same as coal, wood and diesel for his beloved machines. But as Melissa chattered on about recipes for allergen free biscuits and biodegradable wrappers, he thought he might find a way to her heart. Why not make the biscuits and packets with the same recipe? It was time to investigate corn starch and fructose.

Now he was no longer Mr Rust, but Mr Green, inventor of the edible carrier bag and three days ago Melissa had become Mrs. Green. Channel Four was making a documentary about their plans for a perfect Ecohouse with living walls.

But no sooner had the carrier bags become familiar in every supermarket than the first criticisms began to appear on social media. Members of the public no longer had to feel guilty about plastic or litter; discarded sweet wrappers, takeaway boxes and shopping bags would all be eaten by wildlife, from snails to deer. In fact the carrier bags were so delicious, passing dogs were liable to take a bite out of your shopping.

Then came the first news story from the Familyfresh Fairtrade supermarket. Overnight, all the bundles of new carrier bags had disappeared from the store room. The first clue to the mystery came when three large rats scampered across the feet of the store manager. He ran out into the main store, only to see several more rats slip away from the checkouts. The second clue was the remnant of a carrier bag hanging limply, serrated with huge teeth marks.

A meeting of COBRA * was called after pest exterminators made urgent reports of supersized rats, gardeners posted pictures on Facebook of giant snails and a photograph appeared on breakfast television of a fox the size of a deer hound. Malcolm was summoned to reveal the ingredients of his carrier bags…

*Cobra stands for Cabinet Office briefing room A. Cobra meetings are held in Downing Street to plan government responses in times of emergency.