This serial was originally composed of six episodes, but it was deemed too short of content and reduced to five at the last minute. Producer Peter Bryant ordered Mervyn Haisman and Henry Lincoln to abandon writing the sixth episode and script editor Derrick Sherwin rewrote the fifth episode to provide a conclusion. Haisman and Lincoln were not informed of this, or of the BBC's merchandising of the Quarks, which led to their refusal to write for the series again. Subsequently an additional episode had to be penned for the following The Mind Robber, also making that story five parts.
Patrick Troughton was absent during location filming. He was doubled by Chris Jeffries in all location footage.
This was the final Doctor Who story that Patrick Troughton watched during his lifetime. At his request, a screening was held at a convention on the evening of March 27, 1987, before he died of a cardiac arrest the following morning. Troughton reportedly did not think much of the story on its initial broadcast, and asked for the screening to see if he liked it any better nearly two decades later; reports vary as to whether or not he changed his mind about the story afterwards.
Fans voted this the ninth worst Doctor Who (1963) story of the entire run in fan site Outpost Gallifrey's 40th anniversary poll in 2003.