"Whale killer" is the second episode of Walking with Beasts. It shows the time where the mammals began dominate the earth (the Eocene), on land or even in the sea. Most of the episode revolves around a female Basilosaurus, a prehistoric whale, in her battle to sustain and then raise her calf.

Other animals shown are Embolotherium, Andrewsarchus, Moeritherium, Dorudon and Physogaleus.