Jimmy Doherty, Kate Quilton and Matt Tebbutt reveal more unusual, intriguing and surprising secrets behind the food we eat. Matt heads to Italy to find out how supermarkets can sell pesto at an affordable price when the traditional ingredients are expensive. He visits one of Europe's biggest pesto producers and takes a remarkable trip to the bottom of the Mediterranean Sea, where basil is being grown in an extraordinary underwater farm. Jimmy wants to know why so many cockles in our supermarkets come from abroad, when he always thought of them as a quintessentially British staple. He visits Wales and the Wash, where two very different stories unfold, and witnesses a remarkable method of cockle gathering that has to be seen to be believed. And why do marshmallows go so crispy and melty at the same time over the campfire? Kate takes a colourful trip around Europe's largest marshmallow factory to find out.