Gregg Wallace is in London at Europe's largest biscuit factory, where they produce 80 million biscuits every day. He follows the production of chocolate digestives, from the arrival of 28 tonnes of flour right through to dispatch. Along the way, he discovers that the biscuits are shaped by a bronze roller costing up to ten thousand pounds, and that the chocolate is added to the bottom not the top of the biscuits, meaning we are all eating them the wrong way up.

Meanwhile, Cherry Healey is on the trail of that chocolate. At the refinery in Manchester, she learns that it is transported in heated lorries kept at 50C to stop it solidifying on its way to the factory. She also discovers that this is the most expensive ingredient, at around £2,000 per tonne. And she is in Nottingham University's sensory lab, where she finds scientific proof that dunking your biscuit improves its flavour and that tea is the best liquid to dunk in. Cherry takes to the streets to see if that stacks up in the real world.

Historian Ruth Goodman investigates the link between biscuits and digestion. She finds references in Samuel Pepys's diary to biscuits being a cure for flatulence and digestive discomfort and discovers that in Victorian times it was thought that biscuits could cure everything from typhoid to scarlet fever. She also takes a look at an antique biscuit baked at the beginning of the 20th century - one of Huntley and Palmer's notorious army biscuits. These dry hard biscuits were supplied as rations to five million British soldiers on the front line in the First World War.

Gregg Wallace explores the Grimsby factory that processes 165 tonnes of fish a week and produces 80,000 cod fish fingers every day. Cod arrives at the factory as compressed blocks of frozen fish. The blocks weigh exactly 7.484 kilos, which is a standardised measure in every fish factory right across the world. Gregg watches as each block is cut into 168 naked fish fingers which are then floured, battered and breaded, ready for a quick 45-second trip through the fryer. He also helps take delivery of 25 tonnes of liquid nitrogen, used to flash freeze the fingers at minus 15 degrees C. But Gregg is amazed to discover that the fish inside the finger remains frozen through every stage of production, right up to the moment you cook it at home.

Meanwhile, Cherry Healey travels to Grindavik in Iceland where they land up to 50 tonnes of cod a day. She follows the fish through the processing factory, even trying her hand at gutting the fish. Back in Grimsby, she assists with an ancient method of preserving fish - cold smoking. She learns that the yellow colour of smoked haddock is not down to the smoke but instead is produced by the addition of a natural colouring made from turmeric. Also, just like nine out of ten Brits, Cherry isn't very confident about how to safely defrost food, so she heads to the lab to get the lowdown on bacteria and freezing.

Historian Ruth Goodman is investigating the origins of cod fish fingers. She finds that Bird's Eye were the first to introduce them to the UK, basing them on a US product called fish sticks. They were introduced in 1955 and were an instant hit. 542 tonnes were sold in the first year of sale. That went up by 600% the following year. But the British public had a narrow escape - the original idea was that fish fingers would have been made with the oilier and bonier fish, herring. Ruth's also looking at Britain's original fish-based convenience food: the oyster. In the 19th century, Londoners could buy four for a penny, but an outbreak of food poisoning after a banquet in November 1902 caused a national scandal and their popularity plummeted.