Alan Bromly departed in the last block of shooting because of an irreconcilable conflict with Tom Baker, so producer Graham Williams took over. According to accounts by visual effects designer Colin Mapson and assistant floor manager Val McCrimmon, Alan Bromly simply didn't understand how to direct the programme efficiently - and wasn't interested in learning. Consequently, he was removed from the project by Graham Williams. Although Barry Letts had preceded Williams as an "emergency director" (for "Inferno" when the contracted director, Douglas Camfield, was hospitalised with heart problems), this was the only known instance of a producer stepping in for a director that he had been forced to fire. Mapson, longtime veteran of the programme, flatly called it "without doubt, the most disastrous Doctor Who (1963) I've ever been involved in". When production finally wrapped, crew members were presented with T-shirts saying, "I'm Relieved the Nightmare is Over".
This is one of the few Fourth Doctor stories to have a strong moral message, in this case against drug abuse and the illegal drug trade. The drug in question was originally going to be called "xylophilin", or "zip". However, Lalla Ward was worried that the name would sound appealing to children, so it was changed to "vraxoin" instead. However, K-9 still mentions vraxoin as having the scientific code "XYP".
On the DVD commentary, Bob Baker and Lalla Ward both felt that Lewis Fiander's performance was completely over-the-top and, along with the quality of the Mandrels, let the story down. However, they thought David Daker was brilliant. Colin Mapson thought the studio lighting was too bright and the show lacked atmosphere as a result.
Lalla Ward was keen to omit anything which might appear to glamorize the narcotics trade. As a result, various uses of drug language were amended to sound less appealing or exciting. Most notably, the drug at the center of the story was originally called "xylophilin" or "XYP" and nicknamed "zip". This was changed to "vraxoin" (or "vrax" for short).
The glow inside the mandrel costume's eyes was achieved by slipping glow sticks inside of them.