David Attenborough tells about the in-depth exploration of the undersea world, from the known to the unknown, all the secrets of the sea are revealed. We follow the largest animal on Earth, sink to the depths of the ocean, see the food chain in action and discover the bizarre eating habits of dolphins. In addition, Attenborough asks the question: do you know where the fish from the fish trade comes from and do you have any idea that in some cases these are endangered species?