This time, we meet a girl with two hearts, a man who can sing two notes at once, a woman who can bend in amazing ways, a girl who is allergic to everything and a man who can run 350 miles without stopping. These remarkable cases reveal the secret inner workings of our bodies - the ultimate piece of natural engineering.

In this episode, Gabriel uncovers the cases of an engineer who fixed his own heart, a toddler whose bones were repaired before he was even born and a girl whose immune system attacked her own brain. We meet a man who can taste words and find out how his condition is helping develop new ways to enable blind people to navigate and even recognise colours. And we encounter a man who was immobilised by MS but can now cycle and scuba-dive thanks to a pioneering new treatment that has reversed his disease.