Most of the stories about the Third Doctor were set on Earth and feature UNIT and the Brigadier heavily. While not as ubiquitous in following years, he appeared alongside every subsequent Doctor in the original television series run, excluding the Sixth Doctor, with whom he appeared only in the 30th anniversary special, "Dimensions in Time" in 1993. Although Lethbridge-Stewart first met the Doctor in his second incarnation, he also met and worked with the First Doctor in the opening serial of the 10th anniversary season, "The Three Doctors" and again in the 20th-anniversary special, "The Five Doctors". He eventually retired from the military to teach mathematics at an English public school in 1976, as seen in "Mawdryn Undead" (1983). The Brigadier and the Sixth Doctor, as well as later incarnations of the Doctor, have been paired in numerous spin-off productions (see Other appearances).