Clips survive due to cuts made by the Australian censor.
This episode was wiped by the BBC and no copy of it is known to exist.
This episode takes place in England in 1968.
When one of the engineers is checking the gas flow in one of the sub-stations, there is a notice board in the background with a picture of the Master, as played by Roger Delgado. The Master would not make his appearance until the time of the third Doctor story arc 'The Terror of the Autons' in season eight of the classic era Doctor Who.
Margaret John is one of 39 performers to acted in both the 20th Century series and the 21st Century series, plus she is among half a dozen to have acted in both the black and white era of Doctor Who and the 21st Century era of Doctor Who.