Evolution

"Someone is tampering with the fabric of the human cell," the Doctor said darkly, "perverting its secrets to his own dark purposes."

Sarah wants to meet her fellow journalist Rudyard Kipling, and the Doctor sets the co-ordinates for England, Earth, in the Victorian Age. As usual, the TARDIS materialises in not quite the right place, and the time travellers find themselves pursued across Devon moorland by a huge feral hound.

Children have gone missing; at the local boarding school, the young Rudyard Kipling has set up search parties. Lights have been seen beneath the waters of the bay, and fishermen have been pulled from their boats and mutilated. Graves have been robbed of their corpses. Something is going on, and Arthur Conan Doyle, the ship's doctor from a recently berthed arctic whaler, is determined to investigate.

The Doctor and Doyle join forces to uncover a macabre scheme to interfere with human evolution — and both Sarah and Kipling face a terrifying transmogrification.