This episode takes place in London on July 20, 1966.
The only surviving footage from this episode was recovered from Episode 6 of The Wheel In Space the following year. Whilst initially believed to have been the same clip from the following episode, upon more thorough examination conducted in 2004 it was discovered that the clip contained a few more frames of footage.
This episode was wiped by the BBC and no copy of it is known to exist.
As with The Power of the Daleks, Terry Nation was unavailable to write this serial, so he entrusted the task to David Whitaker.
Voted by fans as the eighth greatest Doctor Who (1963) serial in Outpost Gallifrey's poll in 2003 to celebrate 40 years of the series.