 Filming of the episode began on 13 March 1967.The incidental music for both this episode and "Avalanche" was recorded in a four-hour studio session held on 11 June 1967 with a 15-member orchestra.Dialogue in this episode states that only high-voltage electricity can kill Mysteron agents, yet in both earlier and later episodes (including "Winged Assassin", "Manhunt", "White As Snow" and "Shadow of Fear") they are shot dead with conventional handguns.