Documentary telling the extraordinary story of Koko, the only 'talking' gorilla in the world, and her lifelong relationship with Penny Patterson. Project Koko started as a PhD project to teach sign language to a baby gorilla, but as Koko began to communicate with Penny, an intense bond formed between them. Penny has now been with Koko for over 40 years and claims Koko can reveal fresh insights into the workings of an animal's mind. Koko's unique life with Penny has been filmed every step of the way. Over 2,000 hours of footage chart the most dramatic moments - Penny's battle to keep Koko from being taken back to the zoo in which she was born, Penny's clash with academic critics who doubted her claims and the image of Koko mourning the death of her kitten.