Cameras return to the Zoological Society of London to explore the changing role of zoos in the 21st century, the struggle to protect endangered species, and the extraordinary relationships between keepers and their animals. London Zoo’s £4 million new showpiece, Tiger Territory, is being made ready for its two stars, Sumatran tigers Jae Jae and Melati. Cardiologists are operating on chimpanzees to establish why so many of them die of heart disease. Burmese python Bertha is being treated for a respiratory infection, a limping Humboldt penguin is anesthetised and X-rayed, and an ultrasound is carried out on a pregnant Giant Galapagos tortoise. In the zoo’s rainforest, keepers work out how to keep the Tamarin monkeys from stealing the sloth’s food. Tiger Territory gets a royal opening by the Duke of Edinburgh, and the two big cats mate – which may mean the zoo will have its first tiger cubs in 15 years.