At the time it was written, Frazer Hines was considering leaving the show. He finally signed on for the season, apparently at Patrick Troughton's request. This meant Brian Hayles had trouble writing the script, a problem made worse when he was asked to leave the Doctor out of part four. He failed to complete the script and Terrance Dicks rewrote much of the story. He brought in the Martian fleet and removed a sub-plot in which Gia Kelly was hypnotised.