The long-running character of Lethbridge-Stewart (better known by his later rank, the Brigadier) makes his first appearance on the program here. However, the exposure is limited to a view of his boots as he walks through the underground, and they are worn not by the actor Nicholas Courtney, but by an extra named Maurice Brooks.
This episode was wiped by the BBC and no copy of it was known to exist, until 2013 when it was recovered from Nigeria.
The Doctor does not appear in this episode (besides at the start, in a repetition of the previous installment), yet his name still appears in the credits. Patrick Troughton was on vacation that week, and thus the Doctor was conveniently written out of the episode, as was common at the time.
A copy of this episode was discovered in 2013, restored and released to iTunes late in 2013. A DVD is due to be released in the UK in February 2014.
This episode takes place in the London Underground in 1975.