It is 36 million years ago, and mammals have taken over the world. They are no longer small furry animals living under the shadow of dinosaurs or killer birds, they have grown enormous and dominate the planet. This is an era of animals like Andrewsarchus, the biggest mammal carnivore ever to walk on land, and the Brontotheres, small brained bulldozing herbivores. It is in the sea, however, that the most monstrous mammals of all can be found. This programme follows the fate of a female Basilosaurus, a huge serpentlike early whale. She is nothing like the gentle filter feeding whales of the 21st century. Four times the length of a great white shark, with jaws to match, she is every inch a killer.