It was during this season that the Doctor cut ties to UNIT (after Terror of the Zygons, the Brigadier would not appear again until Mawdryn Undead in 1983, and The Seeds of Doom marked the last occasion to date that the Doctor was explicitly said to be working for UNIT - as opposed to working with them as occasions warranted). Several of the serials in this season, particularly Pyramids of Mars and The Brain of Morbius, are among the best-known of the 1970s-era storylines. Though it was aired as a part of Season 13, Terror of the Zygons was actually filmed as part of the previous season; this holdover allowed the series to switch back to an autumn season start.