Reports are coming in to UNIT of trains emerging from a railway tunnel in Sussex, their passengers and drivers dead. The Doctor elects to drive a train through the tunnel himself, but when he emerges Jo sees to her horror that he is covered in ice. Something in the tunnel has driven him close to the point of death. What can it be?
to be added
- Sgt. Benton's brother worked on the railways.
- Mike Yates' father recited the poem "The Tyger" to him as a boy.
- Jo compares the folly to the castle on Peladon. (TV: The Curse of Peladon)
- Jo recalls her encounter with Azal in Devil's End. (TV: The Dæmons)
- Yates mentions that he served in the regular Army before joining UNIT. (AUDIO: The Rings of Ikiria)
- Yates refers to having fought "tentacled monsters". (TV: The Claws of Axos)
- After the Doctor notes he may have been forced to regenerate in the tunnel, the Brigadier comments, "Thank Heavens for small mercies. Three of you was quite enough". (TV: The Web of Fear, The Invasion, The Three Doctors)
- The Doctor is once again angry at the Brigadier for destroying an innocent creature with explosives. (TV: Doctor Who and the Silurians)