The building Alec is supposedly looking at as part of the construction of the studios is Northwick Park Hospital in Harrow
This is one of two episodes (the other is The Psychobombs (1970)) that does not begin with the standard opening sequence.
This is Shane Rimmer's (CIA Agent) third and final appearance on the show. Rimmer had previously worked with creator Gerry Anderson, providing the voice of 'Scott Tracy' in the supermarionation television series Thunderbirds (1965).
Two of the technicians setting up S.H.A.D.O HQ will eventually crew the Skydiver submarine.
The rolling end credits are accompanied by a melody other than the usual show theme.