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.