While visiting his mother, Lethbridge-Stewart is a little perturbed when Harold Chorley calls to ask for his help. A train from Bristol has gone missing, and Chorley is convinced it has something to do with the Keynsham Triangle, where over fifty people have vanished without trace since the early 1800s. Elsewhere, Anne Travers is coming to terms with a loss in her family, and sets about preparing for a funeral. However, news reaches her that both Lethbridge-Stewart and Chorley have gone missing, and her help is required to find them. And, hopefully, solve the mystery of the Keynsham Triangle. What connects the missing train to the Triangle, what has it got to do with a Wren from the 1940s, and just why does it appear that Lethbridge-Stewart and Chorley are in the village of Keynsham in 1815?