So he wasn't a regular who turned up in practically every story then? I thought, because I have heard people talk about 'The UNIT Years' and things like that, that for a long time during the classics they were all in every story. I know it started in the Third Doctor era, but I thought it went right though to Five or something.
When were the UNIT years? Why are they so called? And how many years were there?
UNIT was first introduced in The Invasion, right at the end of Troughton's era (late 1968). From Pertwee's debut in 1970, The Brig and UNIT were in every story in 1970, all but one in 1971, two out of five in 1972 & 1973 and three out of five in 1974 (though the Brig only made a cameo in The Time Warrior). When Tom Baker took over, they were in one story each of the first two series. UNIT featured in two further stories, but the Brig wasn't in them. He then reappeared in two stories in 1983 and one in 1989 Quite a few stories in "the UNIT years" didn't feature either UNIT or the Brig, but there were usually a lot of references to them, since the Doctor and the companions (mostly) worked for UNIT.
