Planet of Giants was the first story of the second season, back in October/November 1964. The original Tardis crew arrive in a contemporary Earth (for the first time since An Unearthly Child) but find themselves miniaturised through some kind of dimensional glitch. It has a bit of a duff reputation among fans but I rather enjoyed it. There are two plot threads, the time travellers trying to extricate themselves from their predicament and the nefarious dealings of an insecticide manufacturer with whom they have unwittingly become entangled. It all worked for me, particularly because the sets are fantastically well designed and convincing.
Am I right in thinking that this was the only ever three-part story? (Edited to add: Until 1987, as loveandgarbage points out.)
Looking through my reviews, I realised that this was the only story featuring Susan that I had not previously seen. I am tempted to do a piece about her as I did for Dodo Chaplet a while back. (I've also seen/listened to Victoria's complete arc, but can't bring myself to write her up.) But I should rewatch the last three eps of An Unearthly Child first, which I only saw in 1981.
The Time Warrior was the first story in the eleventh season of Doctor Who, over December 1973/January 1974. More significantly, it was the first outing for Elisabeth Sladen as Sarah Jane Smith, a role she played until October 1976, the longest continuous run of any companion (and longer than some Doctors had on screen). (Reprised, of course, in 1981 in K9 and Company, 1983 in The Five Doctors, in various Big Finish and other spinoffs, and last year in School Reunion; now getting her own TV series at long last.)
She gets a good introduction, stowing away in the Tardis to investigate the disappearance of scientists, who as it turns out are being kidnapped by time machine by an alien Sontaran who needs them to repair his spaceship which has crashed on Earth in the Middle Ages. (Of course, when they meet again in School Reunion, the Doctor is once again pretending to be Dr John Smith; not, as we now know, for the last time either.) I felt she was a bit screamy compared with the Sarah Jane Smith we came to know and love later on, but in contrast with the awful Jo who came before she is a vast improvement.
There's also an interesting conversation in Episode 2 between the Doctor and the Sontaran commander Lynx with significant continuity implications. Apparently this was the first time that the Doctor's home planet had been named. But it's also interesting that the Sontarans have been considering it as a military target, a plan which comes to fruition in The Invasion of Time in 1978.
Anyway, not one of the great Robert Holmes stories, but not bad at all.