This episode was wiped by the BBC. Prior to its discovery in 2013 no copy of it was known to exist. Though before it could be recovered episode 3 was stolen from the TV studio in Nigeria.
All the missing episodes from 'The Web of Fear' were recovered from a TV station in Nigeria in 2013. They have been digitally restored and made available on iTunes, except this episode. It disappeared before it could be returned to the BBC. To this day, the whereabouts of this episode remain unclear.
Nicholas Courtney makes his first appearance as the long-running character Lethbridge-Stewart, here a colonel in the regular British army, but better known as "the Brigadier" of UNIT.
When Jamie and the driver Evans stop in the tunnel so that Evans can buy from a vending machine, the chocolate bar he picks is "Camfield's", conveniently named after Douglas Camfield, the director of this series and many other Doctor Who series.
Whilst this marks Lethbridge-Stewart's first appearance, his first meeting with The Doctor takes place offscreen.