This story is the first to be recorded using colour studio cameras. The previous serial, "Spearhead from Space", was the first in colour, but was shot entirely on location (i.e., outside the electronic TV studio), and on film (as opposed to videotape, the standard method for recording "Doctor Who"). Due to the move to colour, the production team made use of a technique known as Colour Separation Overlay (CSO, or Chroma key), which allowed images to be superimposed over each other using colour separation. This was used extensively in the series for many years, beginning with this serial.