Gregg Wallace is in France at an enormous foundry that produces a cast iron pot every five seconds. He follows production of casserole dishes from the arrival of 20 tonnes of crude iron right through to brightly coloured orange pots. Along the way, Gregg tests his mettle by taking a sample of molten iron at 1,550 degrees Celsius. With only a heatproof visor and gloves as protection, he dips a ladle into a bubbling cauldron and pours the white-hot sample into a tiny mould. He also discovers that the coloured enamel they protect their pots with is made from glass.

Meanwhile, Cherry Healey is in South Africa visiting one of the largest iron ore mines in the world. Nine miles long by three miles wide, it produces a staggering 670,000 tonnes every day. Cherry rides in one of the biggest dumper trucks in the world. Seven metres tall and packing 2,500 horsepower, it collects 65 tonnes of freshly mined rock and dumps it in to a processing plant. Days later, the iron ore is taken away from the mine in a two-mile-long train. And Cherry is rooting out the science behind cooking the perfect casserole. It turns out that when it comes to cooking time, longer isn't always better.

Historian Ruth Goodman takes a journey through time, learning how one-pot cooking evolved. From communal ovens during the industrial revolution through to 1970s slow cookers, technology influenced how people prepared simple meals. Ruth also visits the birthplace of the industrial revolution in Shropshire to discover how casting iron in sand moulds democratised our kitchens by producing affordable cookware.

Gregg Wallace is in Ireland at an enormous liqueurs factory that produces 540,000 bottles a day. He follows the production of cream liqueur from the arrival of maize to make Irish whiskey right through to dispatch of the finished liqueur. It is the show's longest ever production timeline, taking more than three years. Along the way, Gregg learns that it is the barrels whiskey is matured in that create around half of its flavour and discovers that a milk protein is the secret to mixing cream and whiskey together.

Meanwhile, Cherry Healey is at the plant where 85 per cent of Ireland's bottles and jars are recycled. They process 500 tonnes every day. Cherry also investigates the science behind aperitifs. There is nothing special in these beverages that stimulates appetite - it is something common to all alcoholic drinks. Cherry puts it to the test with a team of rugby players and discovers they eat 8 per cent more – or an additional 320 calories – when alcohol is involved. Cherry is also getting a lesson in the rules of whiskey, learning that single malt must be made from 100 per cent malted barley from a single distillery, whereas bourbon must be 51 per cent maize, and blends can be the product of a mix of grains from different distilleries. The one thing they have in common is that they must be matured in wooden barrels for a minimum of three years in order to be called whiskey.

Historian Ruth Goodman is getting spiritual with the history of liqueurs. She learns that their origins are to be found a world away from funky downtown bars. She visits a former monastery and discovers that the drinks were invented by monks looking for the elixir of life. Ruth also visits a distillery in Ireland, where she learns that 100 years ago Irish whiskey held an astonishing 60 per cent of the global whisky market. Today, it is just 5 per cent. This drop was largely due to resistance to adopting the modern column still method of distillation.