Written by Christopher Priest, the opportunity to write this four-part story was offered to Priest after his previous script, "Sealed Orders", had been cancelled. The scene breakdown was commissioned on 5 December 1980 and the scripts on 6 February 1981. Priest's story idea dealt with the 'secret' of what actually powered the TARDIS, in this case fear. Somewhere hidden inside the TARDIS was the one being the Doctor feared above all others, and the psychic tension between the two of them produced the energy to move through space and time. The story involved the Doctor having to confront and ultimately defeat this fear, and was designed to write out the character of Adric. After hearing nothing from the production office with regard to his completed scripts or his payment for them, Priest made contact with John Nathan-Turner. He was told that the scripts were unusable and that he would not be paid. After a bitter dispute Priest was paid and both Nathan-Turner and Eric Saward forced to pen a letter of apology over their treatment of the writer. The script was replaced by Saward's script "Earthshock".