He was "missing" in darkest somewhere and turned up in the last episode that she appeared in (although you only saw the back of his head) and the two drove off into TV land for new adventures....

Tara King took over in that episode as the new avengers girl.

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Indeed the episode was entitled "The Forget-Me-Knot" which not only introduced us to Tara King but also to Mother.
I still find it quite poignant in that episode when Mrs Peel whispers to Steed " Always keep your bowler on in times of stress and watch out for diabolical masterminds" with Steed's reply "Emma............thanks" the only time he's ever referred to her by her christian name.
We then see her jump in the motor with the old man who of course is a ringer for Steed...........apologies for the anorak post.......

There's no sun, the shadow of the wizard.......