04x02 - The Gravediggers
This is probably the best bondage scene in The Avengers. Not only it features lovely Diana Rigg as Emma Peel dressed in a really sexy leather outfit and tied up and gagged two times, but we can also enjoy a brief scene of Caroline Blakiston who plays a nurse who gets bound and gagged by Emma.
I guess no more description is needed. Sorry for so many pictures I simply couldn't resist.