Spearhead From Space may have been the start of the UNIT era of the show but The Invasion was a story the production team designed to foreshadow it. It was basically to Spearhead From Space what Silence In The Library/Forest of the Dead is to The Eleventh Hour: introducing audiences to the type of stories that will be told in the upcoming new era. In Silence In The Library/Forest of the Dead we're introduced to the notion of time travel playing with the way the Doctor meets people. Then in The Eleventh Hour, we see time travel utilised so the Doctor meets his new companion Amy Pond as a child first, before meeting her as a grown-up companion. In The Invasion, meanwhile, audiences are made familiar with the concept of the Doctor working alongside UNIT to stop alien invasions.

The Invasion sees the Doctor (Patrick Troughton), Zoe (Wendy Padbury) and Jamie (Frazer Hines) arrive in London looking for Travers after being fired at in space by an unknown entity. They arrive at Watkins' house to find model Isobel Watkins (Sally Faulkner), who tells them Travers has moved to America and her father Professor Watkins is staying there instead. It's not long before the TARDIS crew are drawn into a mystery concerning Watkins' mysterious disappearance (he has been missing for weeks). They learn that he works for International Electromatics and the Doctor and Jamie decide to investigate the company. After being gassed by two men watching via a video link, Tobias Vaughn (Kevin Stoney) orders security chief Packer (Peter Halliday) to take them to his office. Before long, the Doctor and Jamie cross paths with UNIT (who are investigating International Electromatics) who they find is led by their friend Alistair Lethbridge-Stewart (Nicholas Courtney) from The Web of Fear, who is now a Brigadier.

Episode One of The Invasion sees the TARDIS crash in a field on Earth when it comes into contact with missiles. The Doctor (Patrick Troughton), Jamie (Frazer Hines) and Zoe (Wendy Padbury) decide to find Professor Travers to help them prepare a circuit from their ship. They ring the doorbell. A young model - Isobel Watkins (Sally Faulkner) answers. She tells them that Travers is in America with his daughter Anne and her father Professor Watkins is staying in the house. Watkins works for International Electromatics and has been missing for weeks. The Doctor and Jamie decide to pay a visit to the company where he works believing Travers can help them with the repair of the circuit. In episode four, the Doctor rescues Jamie from a container containing what is later revealed to be a Cyberman. Zoe and Isobel are brought into the building by Tobias Vaughn's (Kevin Stoney) guards and taken prisoner so that they can be used as bait. Tobias threatens to the Doctor that Zoe will face the consequences if he and Jamie don't hand themselves in. UNIT save the Doctor, companions and Isobel with a helicopter: Zoe and Isobel climb through the window in the room they are imprisoned in and the helicopter pilots throw down a rope ladder for the group. The Brigadier tells them that he suspects that the ship found on the dark side of the moon and International Electromatics' deep space transmitter may link into current UFO sightings. The Doctor thinks the crates they discovered links in and decides he must go back to International Electromatics with Jamie to investigate.

Kevin Stoney is amazing as Tobias Vaughn. I have absolutely no idea why I forgot to say that for my review of The Invasion as he is without a doubt one of the show's best one-off villains. He has a very strong sinister presence and the way he plays Vaughn with such a cold, quietly menacing demeanour is captivating to watch. Of course, in two of these episodes we only get it through animation but the animators did a good job at capturing Kevin Stoney's mannerisms from the other episodes.

In fact, some may say the animators here have an advantage over, say, the animators of Power of the Daleks - who only have a few surviving clips to go on. That doesn't stop The Invasion's animated episodes from being top-class animation though. It is somewhat unsurprising that the animation is so strong given that the episodes were made by the team behind Danger Mouse - Cosgrove Hall - but everything looks so rich, crispy and detailed. Granted, it's not to the quality of Disney cartoons but animated Doctor Who episodes are never going to have that kind of budget. The Cosgrove Hall animations have been the best animated representation Doctor Who is ever going to get for a while (although judging by the trailer for The Power of the Daleks it looks like that's soon going to change). The animation here is so engrossing to watch that you forget about the Cybermen's absence until the end of episode 4 (despite this being a Cyberman serial).

What really works about the animation is the genius decision to animate it in black and white. With the releases of The Reign of Terror, The Moonbase, Planet of Giants and The Ice Warriors it is easy to forget that black and white may not have been the standard for animated missing episodes of Doctor Who before 2006. It is thanks to the decision made by Cosgrove Hall that we get these animations in black and white. Black and white is extremely effective for missing William Hartnell and Patrick Troughton serials not just because the real episodes were black and white (making the animation authentic) but also because it's very atmospheric. One of the things I enjoy about watching Hartnell and Troughton era Doctor Who is the atmosphere that the episodes not being in colour provides. An Unearthly Child wouldn't be the same colourised - and the same goes for The Invasion. Black and white adds to the 'hide behind the sofa' value of Doctor Who; it gifts a certain chill to proceedings that wouldn't be there if they had been filmed in colour.

One of the things you want from an animation recreating a missing episode like this is for the characters to look like the actors they represent - and this is certainly true here. They look exactly like them, meaning that watching the animation mixed with the live-action isn't a distracting experience at all.

The best character design in my opinion is that of the second Doctor. The animated Doctor basically is Patrick Troughton here and from pictures of recent animation efforts I don't think any other company has even come close to providing a better likeness than here...that was before the Power of the Daleks announcement. That's right: the animation company behind The Power of the Daleks has somehow managed to improve on Cosgrove Hall's second Doctor:

The thing I find most incredible about The Invasion animations though is the quality of the off-air audio recording. It sounds as though it was recorded today rather than nearly fifty years in the past. The sound is crystal clear and you can hear everything that is said perfectly. In fact, I'd go as far as to say it sounds almost as though it was recorded especially for the animated episodes. There's no obvious sound problems, no sudden dip in the quality between the audio of the surviving episodes and the missing episodes' audio. It's simply a perfect recording and helps contribute to what is arguably the best animated representation of missing Doctor Who episodes so far...until Power of the Daleks, of course, which is obviously going to be awesome.

The Invasion is a very bold and successful story for the show as despite the fact that it takes four out of eight episodes for the story's monsters to turn up (the Cybermen) it never drags or feels particularly slow. The entire story is immersive and well-paced, especially compared to something like The Hungry Earth/Cold Blood from the new series (which feels slow and drawn-out). I actually would have liked to have seen more Decond Doctor stories like this as he works extremely well with UNIT. Patrick Troughton and Nicholas Courtney have so much chemistry together and it's a shame Nicholas Courtney didn't appear in more Patrick Troughton serials.

The most noticeable thing about this serial is that it feels as though it's among the higher-budgeted of the classic series serials. Like The Enemy of the World, it has a helicopter but there are also many different locations such as the Watkins' house, Tobias Vaughn's office, UNIT's mobile HQ and the sewers. The serial was also filmed in London, which no doubt would have cost a lot at the time. I'd say most of season six's budget was probably spent on this serial. If so, it definitely shows and it makes it a good serial for any Whovians who have currently only watched the new series but want to give the classic series a go. They won't be as distracted by the shoestring budget as with other serials such as The Invisible Enemy or The Krotons.

The one thing that might put off Whovians who have only seen the new series is something I've already mentioned: the fact it takes four episodes before the Cybermen show up. Personally I would argue this is one of the serial's strong points. The slow build-up invasion is very effective for this serial as it allows the focus to be more on the sinister Tobias Vaughn than if the Cybermen had been in the story from the start and subsequently overshadowed him. Once they do return, it's more than worth the wait. You get some truly iconic shots, such as the Cybermen marching down the steps of St Paul's Cathedral or the Cybermen in the sewers. These are shots that have been a huge inspiration for the new series, with the Cybermen St Paul's Cathedral shot rehashed in Dark Water/Death In Heaven.

It's a shame Sally Faulkener's Isobel Watkins didn't become a companion after this serial as she is brilliant. I would have loved to have seen Isobel travelling in the TARDIS alongside Jamie and Zoe; it would have offered a nice dynamic. Perhaps the Second Doctor  could have got frustrated each time she spends too long deliberating over what to wear before leaving the TARDIS? Whatever they'd have done, Sally Faukener clearly has a lot of chemistry with Wendy Padbury in particular and it is always nice to see a young strong female character (Isobel bravely takes pictures of the Cybermen in the sewers) comparable to a modern day contemporary character than one from the 60s. The classic series did have a good track record for strong female characters - Barbara Wright, for example - and Isobel continued the show's trend at the time to be ahead of its time. Of course, the show's main cast are all on top form in this serial as usual - especially Patrick Troughton when he expresses his annoyance at the answering machine at International Electromatics - and Nicholas Courtney is brilliant but Sally Faulkener steals the show as Isobel Watkins.

Overall, The Invasion is one of the show's best serials to feature the Cybermen. It is expertly paced and features one of Doctor Who's all-time greatest one-off characters in Isobel Watkins. It is a shame that she never became a companion as she is a brilliant character and Sally Faulkener displays a lot of chemistry alongside the show's cast in the story. I looked to see if Big Finish have ever used the character but unfortunately they haven't. Hopefully one day they will as she deserves to return in some form.