I didn't really want to get to the end of the DVD of the last few episodes of the 1967 season of the Avengers, because I didn't want to see Mrs Peel leave. I've nothing against Tara King (Linda Thorson), but my memories are that the series got quite a bit sillier after she joined Steed - and she was definitely an assistant, rather than a partner.
However, the penultimate episode, Mission....Highly Improbable, had the added bonus of Nicholas Courtney playing the head of security at the secret scientific base - in a uniform and bright red motorcycle helmet that made him look like Captain Video! - and Francis Matthews as one of the scientists - the voice of Captain Scarlet, and Paul Temple in the TV series. So I took great delight in seeing The Brig chatting to Steed as they attempted to solve the mystery of the disappearing Rolls together.
And thus fortified, I went on to watch The Forget-Me-Knot, in which just about everybody loses their memory at some point, but even when they don't remember who they are, Steed and Mrs Peel manage to thwart the bad guys. The episode actually introduced Tara King really well, showing her to be resourceful and quick witted, and she joined Steed in the end titles. At the time, The Avengers was a very popular show with a fairly big budget - they had a helicopter for Murdersville! - and they obviously took great care with the replacement of the co-star.
This was also, I think, the first mention of Mother, and a clear statement that Steed was a secret agent. In earlier episodes, it's usually left vague, and he introduces himself as being "from the Ministry" or some such, usually with an eye-poppingly high security clearance. I'd forgotten Mother was in a wheelchair, too (was Ironside on TV in the UK by then, or was that later? The first season was certainly filmed in 1967).
The final scenes, when Mrs Peel's husband is discovered alive, and drives off with her (wearing a bowler hat almost exactly like Steed's) are really very touching. Patrick McNee confessed in the extras that he had gone off to his dressing room for a little cry after Diana Rigg had kissed him goodbye.