Finally we have reached the final segment of the Key to Time - found in the Key To Time season finale The Armageddon Factor. An interesting note to make about The Armaggedon Factor is that Part One was the five hundredth episode of the show. Unlike The Stones of Blood, The Armageddon Factor is a much more memorable milestone story too (although it does have the advantage of being the finale).

The Armageddon Factor sees the Doctor (Tom Baker) and Romana (Mary Tamm) arrive on Atrios: a planet that's at war with neighbouring planet Zeos. They are discovered by the Marshal (John Woodvine), Atrios's commander in the war and decide to flee back to the TARDIS. However they find the TARDIS has gone, trapping them on the planet. The Marshal persuades the Doctor to help Atrios in the war. During the story, they meet Princess Astra (played by future Romana Lalla Ward) who is the last Royal Family member of the sixth royal house of Atrios. Meanwhile, one of the Black Guardian's agents - The Shadow (William Squire) - is also looking for the last segment of the Key to Time.

The story feels suitably epic, like a new series finale of the Russell T Davies era. This is the Bad Wolf/Parting of the Ways of the classic series: huge fun to watch, with a certain comic-book feel to the events of the story. I think it's impossible not to be gripped by this story; personally I never lost interest once and I think this would be a good classic series serial to introduce to new fans of Doctor Who. It doesn't matter if they haven't seen previous Key to Time serials: you don't need to know the actual events of the past stories, only that the Doctor and Romana have currently acquired five of the Key to Time segments and the Doctor was sent on a mission by the White Guardian to find them before the Black Guardian does. All of this could probably be picked up by only watching The Armaggedon Factor anyway.

I imagine this serial was one that didn't go down well with Mary Whitehouse as there are a number of dark themes in this story such as nuclear war and death threats. Perhaps the darkest of all however is the real identity of Princess Astra, who is revealed to be the last segment of the Key to Time. Her existence comes to an end when Romana uses her tracer to return her to her true form, meaning that whilst she didn't actually exist she believed she did and still had a life.

It's nice when the show explores such dark themes as it helps add a hint of realism without overstepping the mark with the dark/gritty rubbish that's plaguing Hollywood. Not everything has to be cosy and sometimes it is better for Doctor Who to unsettle its audience.

One of the best elements of this serial is evil K9. K9 works well as an extra threat for the Doctor and Romana when he's being controlled via a device by The Shadow. It's disturbing to see one of the Doctor's companions turn against the Doctor and Romana and it helps to showcase the Shadow as one of the most serious threats for the Doctor to face. This isn't any old villain, he knows exactly what he's doing and how to get it. It also neatly ties in with the sense of loyalty people have with their dogs: the idea that their dogs would never betray them. So what happens when they do? If you can't even trust your pet dog, then who do you trust? Turning K9 temporarily evil was definitely a good idea.

Of course, you can't do a review of The Armageddon Factor without mentioning Drax. Drax's introduction isn't very strong (he literally appears out of nowhere) but he is one of the show's strongest guest characters. He's a renegade Time Lord who was in the same class as the Doctor at the Prydonian Academy. He works as a galactic maintenance man and a death threat from The Shadow forced him to work on the planet Zeos's giant computer Mentalis. Barry Jackson is a revelation as Drax and it's a shame he was never brought back as a recurring character in the television series. Thankfully Big Finish has since brought Drax back with the fourth Doctor audio The Trouble With Drax, albeit not with Barry Jackson (who sadly passed away in 2013) but with Ray Brooks voicing the character.

One thing I would say about The Armaggedon Factor is the conclusion is a little weak. It feels a bit rushed and a little like a slap in the face to what was set up in the first serial (rather than giving the Key to Time to the White Guardian, theydecide to scatter it across time and space, so the Key to Time segments end up exactly as they were at the beginning of the serial anyway). It almost feels like the Doctor didn't properly think it through, because if the segments are once again scatted across time and space, what happens if they fall in the wrong hands - IE the Black Guardian?

I haven't listened to Big Finish's Key 2 Time yet so maybe Big Finish have explored what happened with the segments since the Key to Time's conclusion but it certainly doesn't feel like a strong end to the final Key To Time serial.

Overall, The Armageddon Factor is a spectacular end to the Key to Time arc. It's a story that would make a nice introduction to new fans of the show and features some cool ideas such as an evil K9 and the Doctor's classmate Drax. There are some dark themes but they aren't treated in a dark/gritty way as with the trend in Hollywood movies but help to unsettle the audience. My only real criticism is that the conclusion is a bit disappointing; other than that, The Armageddon Factor is definitely worth your time.