Samantha Briggs was supposed to be a main companion, but Pauline Collins declined the offer, despite personal appeals by Innes Lloyd and even Frazer Hines.
The first episode to feature the new title theme. The previous episode was meant to feature it but due to a production error the previous episode features the original theme.
This episode was wiped by the BBC and no copy of it is known to exist.
In the original script, the Doctor and Ben investigated the hangar but failed to find Polly, being menaced instead by a falling engine (rather than gas), and Ben and Sam then rescued Polly in episode three.
Whilst Anneke Wills appears in this episode, she doesn't portray her usual character of Polly, although she is still credited as Polly.