The Invisible Enemy is a curious case. It introduced one of the show's most iconic companions in K9 (so iconic, in fact, that he was even included in the level pack for LEGO Dimensions seven years after his last appearance in the series) yet his introductory serial is very average.

The story sees the Doctor and Leela arriving at Professor Marius's ship where a three-man crew are under the control of the sentient virus the Nucleus. The Doctor is also infected and must be cloned so they can be inserted inside his body in order to stop the infection. The special effects are usually not one of the selling points of a classic series serial because they are under a shoestring budget but even taking that into account, they are terrible here. The Nucleus looks unconvincing, like a massive prawn; this is probably the one serial you should avoid showing to any new viewers at any cost. When the special effects are bad, you need your story to help those watching to forget it. Sadly, whilst the story isn't terrible it's not that brilliant either; however it does do something interesting with clones of the Doctor and Leela inside the Doctor's brain and the special effects during this sequence are on the better side for a classic series serial.

With an average serial and a terribly designed monster, thank God then that the acting is on point. I think The Invisible Enemy shows Tom Baker at his best; he is brilliant throughout especially during parts three and four and you cannot detect the awkward relationship offscreen between Tom Baker and Louise Jameson at all (they infamously didn't get on well behind the scenes because Tom Baker didn't like the idea of the Doctor travelling with a violent companion as he believed it to be against the Doctor's moral compass). I also liked Frederick Jaegar; it is a shame he didn't become a recurring character as he is an engaging one.

The serial is also nicely paced at four parts. The Invisible Enemy as a story would never be able sustain any more than that; as a six parter it would have really dragged so four parts feels like its natural length and if it had a lesser number it would arguably have had a bigger impact on the narrative forcing it to be tighter and rushed to a conclusion (something that some had a problem with for some of the new series' series 7 episodes - personally didn't bother me but I can see their point in terms of The Power of Three). It feels like it needed the four parts; no more, no less.

Overall, this is a very average classic series serial let down by a terrible monster design and a forgettable narrative. K9 deserved a better introduction than this but at least it feels like its the right length, featured great acting and a character in Professor Marius who really should have been a recurring one (a trend in the Tom Baker era).