I finished a complete rewatch a few months ago. Yes, season three is utter tripe. Even Vaughn, in the extras, brought up the very scene that made me walk away from it for a few months - a "jungle girl" episode where Napoleon ends up dancing the Watusi in a tree-house with a gorilla.
They tried to get the serious spy biz back in season four, though, with mixed results.
I also just finished Girl From UNCLE just the other day. That show never really strays from the Batman campiness, unfortunately. But hey, Stephanie Powers
That's why I never rented it at all-just Season One, Two, and Four. Season Three I'll be getting from Bittorrent, and only a few episodes.
