Critical Mass is alright. There's a fun moment talked about in a lot of the other reviews where the Seventh Doctor does get to be a bit manipulative towards the story's villain that was memorable, but the rest of it is pretty mediocre.  Sophie Aldred does a pretty good job here but definitely improves her impression of the Seventh Doctor over time. Comparing this to a more contemporary performance by her like in the Haunting of Bryck Place is like night and day.