Jo tells the Controller the date on earth from which she arrived was September 13. That very same date later proved pivotal in another British science-fiction series: Space: 1999 (1975). September 13, 1999 was the date the moon broke free of earth's orbit and began its two-season sojourn through uncharted space.
This episode was watched by 10.4 million viewers on its original transmission.
This episode is a rare occasion where the Doctor both holds and uses a gun.