(This is a review of all four parts of the Three Doctors serial, not just part four).
This episode of Doctor Who is a significant one in that it was the first time we saw more than one of the Doctor's incarnations on screen at the same time. We would later see multiple Doctor interaction in the 20th anniversary special The Five Doctors and the mid 80s story The Two Doctors, as well as the non-canon Children in Need special Dimensions in Time.
And for the first two episodes at least, it's a joy to watch! From the moment the brilliant Patrick Troughton enters you know you're in your element as a Doctor Who fan. Troughton embodies the spirit of Who more than any of the other nine actors who've played the Doctor because it was him who gave the Doctor his oft imitated eccentricity and crotchety lovability.
The chemistry between him and Jon Pertwee is brilliant, with the writing cleverly highlighting both the Doctor's enormous ego and initial reluctance to accept help from others, even if that person is a past version of himself. The only shame regarding the presence of past Doctors is the limited screen time of the First Doctor, which was due to the frailty of the ill William Hartnell.
So where does it all go wrong? Around Episode 3, to be precise. The problem with this storyline is that it has one of the most laughably bad, over the top villains in Doctor Who history. The actor who plays Omega chews up so much scenery it's a surprise the TARDIS set and exterior survived intact.
This wouldn't be so much of a problem if Omega wasn't so key to the last two episodes of the story, but sadly, he is. If you really want to have fun with a multi-Doctor adventure, watch The Five Doctors. In many ways the Five Doctors is cheesier, but it has a consistent quality whereas this four parter starts well and then gets very disappointing.