In this episode experts dissect a 65-foot, 60-ton fin whale - second only in size to its 'cousin' the blue whale - that has died after being stranded off the coast of Ireland. It's a race against time as whale anatomist Joy Reidenberg flies in from New York before the animal's decomposition causes it to explode on the beach.

Veterinary scientist Mark Evans helps investigate why the animal died and explores its extraordinary anatomy. Using whale-size machinery, Joy and the team set to work amidst gale force winds, driving rain, blood, intestines, evil smells and freezing conditions. Meanwhile, advancing tides threaten to engulf the whale, as the team struggles to complete the operation.

Beneath the blubber, the whale's unique anatomy holds vital clues to its evolution. Using a combination of dissection and computer graphics, the programme discovers an animal whose closest living relative is the hippo.

Meanwhile, evolutionary biologist Richard Dawkins explains why the whale's ancestors may have taken to the water and the evolutionary problems that had to be overcome to transform a land-based mammal into an animal that swims among fish.