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).

What a fun episode! The beginning is a little slow but it quickly picks up the pace. We've had space viruses before, but never one quite like this.

A small detail but one I appreciated nonetheless was the secretary not questioning the Doctor's name at all. As a nonbinary person with many nonbinary friends, many of whom use nouns as names, I like the idea of that becoming ordinary in the future.

I already adore K9. I love how he and Leela immediately got along the moment they realised the other isn't a threat. They have a surprising amount of things in common. (And who would have expected the Doctor to have a dog leash in his pocket? I'd love to know the backstory for that!)

I think my only gripe with this story is the repetition of Leela being immune because she's a "savage". She's not any less intelligent than the other humans simply because she wasn't taught all the things they were! It's also a return to the thing that annoyed me the most in the early Three and Jo era, where the Doctor looks down on his companion and doesn't seem to actually *like* her. I have gotten a bit of that vibe with Leela but have managed to ignore it so far. I hope this changes in future episodes.

This is part of a series of reviews of Doctor Who in chronological timeline order.

Previous Story: Empathy Games

This is a strong story, from the introduction of K9, to the excellent model shots. I think the only weak point was the pacing.

Professor Marius is an entertaining side character, the Doctor gives him a level of respect that he gives to few people.

The story is a good bit of silly fun, well acted and ok effects.

Next Story: Image of the Fendahl

Baker and Martin’s script is great, they have always been good at strange and psychedelic sci-fi concepts. Unfortunately their (admittedly ambitious) work is let down by the direction. Derrick Goodwin, in his one and only story on the show, does not manage the difficult balance of comedy and action the script gives him. The pacing is totally off, with plenty of awkward pauses throughout the 4 episodes which rip any tension out of the performances and presentation.

There are excellent model shots and of course the amazing creation of K-9 to distract you. The way he is written in as a regular character is clunky, but you’re happy to have him (I hope he gets less noisy though). Some of the humorous characters work - the receptionist woman in particular is inspired. Others don’t. K-9’s original owner is a bit on and off form, depending on the scene - his last words “I hope he is TARDIS trained” were definitely not good enough for the whole story to go out.

Totally off putting makeup on middle aged men that look very uncomfortable in it makes for unintentionally pitiable villains. If these are bad then don’t get me started on “the nucleus”. The downsides are so unfortunate, because at the heart of this story are some great gags and an interesting concept - an infectious disease in space. I’d love to see this story re-done as a new production. Or even as a Big Finish drama. There’s so much to love