This is just alright.  I found the story very mediocre, even if the idea of the main obstacle our heroes face is in the form of a virus. It sounds like a more interesting idea than what production was able to execute.  K9's introduction is fine but otherwise it is a very underwhelming experience, only halfway salvaged thanks to the charisma of Tom Baker and Louise Jameson.  In spite of it kind of being boring, the Doctor and Leela are entertaining enough on their own that The Invisible Enemy is still very watchable, even if it is just as easy to forget.

It's pretty interesting to learn that Baker and Jameson apparently didn't get along much on set.  You really can't tell thanks to the professionalism of these actors.  I found that aspect quite impressive overall because it does feel like those two have good chemistry together on screen.