Both Roger Delgado as the Master and Katy Manning as Jo Grant were introduced at the start of Jon Pertwee's second series.
They would become hugely popular and Manning's eventual departure is probably second only to Elisabeth Sladen's for making grown men cough or find something in their eye.
Then Katy Manning famously became the first Who companion to pose nude, though of course not in Radio Times. (Picture - Radio Times copyright)