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.
Whilst this marks Lethbridge-Stewart's first appearance, his first meeting with The Doctor takes place offscreen.
This episode was stolen after it was discovered in a TV studio in Nigeria but before it could be recovered. It was later sold to a private collector. It is believed that as the first episode to feature the then Colonel Alistair Gordon Lethbridge-Stewart this episode has a high value and is most likely the reason it was stolen.