The Caves of Erith is a Short Trips audio short story read by Stephen Critchlow. It was released as a subscriber-exclusive download, available with subscriptions that included You Are the Doctor and Other Stories.

After some Christmas shopping in Bristol the Doctor and Lucie take a room at a Bed and Breakfast. The next day, Christmas Eve, the Doctor’s plan to attend a lecture at the local university is ruined when the landlady’s son Oscar, a strange boy with a fascination for bats, goes missing. Armed with Oscar’s notes, the Doctor and Lucie follow the boy’s footprints in the snow and end up in a bat-infested cave inside Avon Gorge; here they are captured by a tribe of bat-like people and taken to Oscar, who has set up a makeshift laboratory deep inside the cave system. Oscar reveals that he is working with the bat creatures’ chief, and has deliberately lured the Doctor there to help in his plan to save the ancient subterranean race. Learning that these ‘Eritheans’ intend to release a drug that will render humanity sterile, allowing the bat-people to flourish and reclaim their planet, the Doctor tries to make the creatures see reason and work with humanity; meanwhile, Lucie berates Oscar on his misguided plan to save an endangered species, and tries to make him see that there are better ways to achieve his aims. As the bat creatures fight amongst themselves over what action to take, Oscar attempts to destroy the poison using sulphuric acid; unfortunately this causes a chemical reaction with the caves’ limestone walls, unleashing a cloud of carbon dioxide gas. The Doctor, Lucie and Oscar are forced to escape to the surface, while the Eritheans retreat into the caves. After returning to the B&B, the Doctor arranges for Oscar to have a place on a science ethics course at the university, and gives him a gift of a telescope; in return Oscar gives the Time Lord a picture he has made, of a mother and child from a race he calls ‘Homo Echo’…