Of the major companions only Dodo & Jamie's intros are missing and we have Dodo's first full story.
Katarina's and Sara Kingdom's are also missing but we only have a handful of episodes for them anyway.
And Victoria's Introductory story also has missing episodes.
But, overall, what you have, just watching the complete stories (though a few have now been animated or are in the process of being animated, which makes things better) is a string of Barbara/Ian/and for awhile Susan, and then you have Companions coming or going like crazy.
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?