The Multi-Master story: a concept that has surprisingly never been done on TV before. Until now. Last year Big Finish released the first ever Multi-Master story in The Two Masters, but this is the first time it has been done in the main series. We've had The Day of the Doctor, this is The Day of the Master. The day many Whovians have waited for all their lives. Thankfully this landmark story more than lives up to the hype of the first Multi-Master adventure - I suspect even the most skeptic Moffat hater would find it hard not to enjoy it.

In World Enough And Time/The Doctor Falls, the Doctor (Peter Capaldi) decides to test Missy's (Michelle Gomez) apparent insistence that she wants to turn good, by picking a random distress call and sending her to investigate with the Doctor's companions Bill (Pearl Mackie) and Nardole (Matt Lucas). They arrive on a Mondasian colony ship, and things quickly go awry [spoiler]when the ship's blue-skinned caretaker Jorj (Oliver lansley) shoots Bill. Partially converted Cybermen take Bill's body to a hospital onboard a ship, where she meets Razor (John Simm), a seemingly friendly man who works at the hospital. Later John Simm's Master confronts Missy; he is concerned about his future as a potential ally of the Doctor, and persuades his later incarnation to team up with him and turn the entire colony ship into a giant Cyber-factory.

The Multi-Master interaction in this story is everything you would hope for. It's funny, menacing, intimidatory and totally unhinged (more in a subtle way as opposed to the clown-like antics of Simm's Master in the RTD era). John Simm and Michelle Gomez have so much chemistry, and their interactions are on par with the Second and Third Doctors in The Three Doctors, or David Tennant and Matt Smith in The Day Of The Doctor.

The ending is absolute perfection for a Multi-Master story - the Simm Master kills Missy, Missy kills the Simm Master. Their Multi-Master interaction ends the only way it could: with utter betrayal.

Some fellow fans were not happy with John Simm's portrayal of the Master as insane in the Russell T Davies era. In this story, however, Steven Moffat has wisely written the part more like the classic series style of the Master and John Simm clearly relishes being able to play the role more like Roger Delgado. Several elements of the classic series Master return, including the use of disguisest (The Razor disguise is fantastic - the prosthetics do a great job of disguising John Simm's face beyond recognition), a goatee, and a more suave and sophisticated demeanour. John Simm is allowed to play the Master as the darker incarnation Russell T Davies wouldn't allow - the way he always wanted to play the Master - and his performance clearly improves for it. It's a significant improvement on the Simm Master's previous appearances, and sees John Simm fast become one of the best incarnations of the Master of all time. He almost gives Michelle Gomez a run for her money.

Michelle Gomez still remains my favourite Master. She is the Tom Baker of the Masters in my view - the one who people will look back on in years to come as the definitive Master. Michelle Gomez has given her incarnation so much depth and complexity, and Steven Moffat has written the incarnation with so much ambiguity you have no idea what she's going to do next. She's not a black and white villain, she's so much more than that. She could look to be helping the Doctor one minute, then completely stab him in the back the next. Here the mystery surrounding Missy's motives is played with to great effect in that it's never entirely clear at any given moment whether she's helping the Doctor or she has truly joined forces with her previous incarnation. At one point she tells the Doctor 'I was on your side all along', only to climb a ladder hanging from a ship piloted by Nardole and attempt to leave the Doctor behind with the Cybermen. She claims to Nardole he's dead, and Simm's Master repeats her claims. This ambiguity feels very Hitchcockian, which feels oddly appropriate for a story featuring the Cybermen.

There's a very dark vibe throughout, and the story feels like it's deliberately evoking the Philip Hinchcliffe era of Doctor Who. Missy and the Master have a very disturbing romance - they dance and flirt with each other, and it's clear Simm's Master has feelings for Missy - in a way that feels like it would have almost certainly received an avalanche of complaints from Mary Whitehouse. The romance between these characters is so creepy it feels almost like it belongs in the DC Comics' series Gotham. Indeed, Michelle Gomez is even a part of that universe - although John Simm currently isn't.

The dark vibe continues with the use of the Mondasian Cybermen from The Tenth Planet. I wasn't sure whether they would work brilliantly for HD television, but in this instance I'm happy to be wrong. Their low-budget design makes them all the more scary in HD, and at no point does the design look so low-budget in crystal clear imagery that it looks noticeably low-budget. These Cybermen absolutely work for modern television, and there is a strong narrative reason for why the Tenth Planet Cybermen are there. This is the 'genesis of the Cybermen' - as Simm's Master calls it - onboard a Mondasian colony ship, so this design is needed for the story to work. Some diehard fans may worry that this contradicts Big Finish's Spare Parts, but as this takes place away from Mondas both origins can exist in the same continuity.

I'm glad Steven Moffat decided to do this story, because it is by far the most terrifying the Cybermen have ever been in the new series - and arguably in the show's entire history. World Enough And Time/The Doctor Falls is full-on horror, with scenes such as partially converted Cybermen chanting the word 'pain' as Bill walks down a hospital ward and references to the Cybermen converting children because 'there's less waste'.

At one point, we see a fully-converted Cyber-Bill who believes she is human discover the truth that she has become a Mondasian Cyberman by staring into a mirror; it's a shocking moment that brings to mind classic Doctor Who serials such as The Ark In Space or The Brain of Morbius in its disturbing imagery.

The cinematography is excellent throughout, with the Mondasian Cybermen largely kept darkly-lit or in orangey apocalyptic lighting. They do appear in a bright environment with the solar farm in The Doctor Falls, but the design is surprisingly not hampered by that. They still appear threatening, and no design flaws are exposed when they are in the open.

There's also a great moment where you see Bill in the human form she thinks she still carries and the shadow of her actual Cyber self on the wall.

It's a nice touch that adds to the classic Jeykll and Hyde-esque imagery.

Talking about Cybermen, it's important to note that this isn't just a Multi-Master story but it's also a Multi-Cyberman one. We see more than one variation of the Cybermen, and it's a delight to see them share the screen. The Cybus Cybermen and the Gaiman Cybermen appear alongside the Mondasian Cybermen; neither feel like they steal the spotlight from the Mondasian Cybermen, and the Multi-Cyberman aspect is executed better than the Multi-Dalek stuff in Asylum Of The Daleks. It even works better than the Multi-Dalek element of The Magician's Apprentice/The Witch's Familiar; having more than one variation of the Cybermen is explained through the colony ship experiencing time dilation - time passes faster at the bottom of the ship and slower at the top. This means when the Mondasian Cybermen are at the bottom of the ship they have years to upgrade themselves into the Gaiman variation, whereas for any Mondasian Cybermen who, say, find themselves at the top where time is slower only ten seconds pass.

This two part finale sees without a doubt the best performance Peter Capaldi has ever given as his incarnation of the Doctor. It will be sad to see him go, especially when his acting here shows he has so much more to give. The way he screams 'NO!' in this episode when Missy appears to decide not to stand with him is one of the most powerful moments of his incarnation, and on the strength of this acting abilities here he deserves to win a National TV Award for 'Best Actor' next year. Hell, he deserves to win an Oscar.

As does Pearl Mackie. The Series 10 trailers for Pearl Mackie did her no justice, the promotional material making her performance look rather one-note. She's far from that. Pearl Mackie has been a revelation as Bill Potts, delivering one of the best companion portrayals in the series so far. She is instantly endearing, and gets an utterly fantastic exit from the show. In fact, I'd go as far as to say it's one of the best companion departures the show has ever done.

Bill remains a Cyberman right to the very end, and when her end approaches her essence is saved by Heather (Stephanie Hyam) from The Pilot. The consequences of travelling with the Doctor still remain, Bill is still dead but her spirit continues in puddle form travelling the universe with her true love.

I criticised Stephanie Hyam in my review of The Pilot, but here she does a great job. It's only a small cameo appearance, but Heather feels less void of personality and more like a real person (or as real a sentient space puddle zombie can be). She's not a highlight of the episode by any means, but it is definitely a step-up from her appearance in The Pilot.

Matt Lucas is enjoyable as usual in the role of Nardole, and the character fulfils an important function in the story of helping defend the solar farm against the Cybermen with grenades disguised as apples. His exit feels a bit more random, and noticeably similar to a classic series-style departure.

He falls in love with Hazran (Samantha Spiro), the mother of a group of children, and the Doctor leaves him to help aid their escape to a solar farm on a higher level of the colony ship.

The ending is a little ambiguous in that we have no idea if they manage to survive or if the Cybermen follow them up to the next solar farm and launch an assault on them; it's unfortunate that a character who many have grown to love has been given such an unsatisfactory ending to his story, and I hope that one day Big Finish carry on his story in a Nardole spin-off set.

If David Bradley playing the First Doctor hadn't already leaked, the way this episode ended would have been the most unpredictable cliffhanger ever. Unfortunately the leak means that you are expecting David Bradley to turn up when he does, as we return to where the story began - the Doctor leaving the TARDIS in a snowy landscape (possibly the South Pole from The Tenth Planet) and refuses to regenerate. This time the scene continues beyond that point, and it's really no surprise when David Bradley comes into view as the First Doctor. Whilst it can't be helped that there's a certain irony to David Bradley stating he is 'the original' (no, that's William Hartnell), he does an amazing job in this fleeting cliffhanger cameo. David Bradley is so much like William Hartnell playing the First Doctor here, it's eery. He looks, sounds and acts exactly like the First Doctor we all know and love.

Overall, World Enough And Time/The Doctor Falls is without a doubt one of the greatest Doctor Who stories of all time. It's Peter Capaldi's Caves Of Androzani, demonstrating the sheer quality and darkness that classic series serial is known for. This delivers exactly what you would expect from the first Multi-Master story, and is possibly the most unsettling the Cybermen have ever been. The Cybermen have never been a greater threat, and there has never been a better exit for a companion than the departure Pearl Mackie's Bill gets here. If any story was to challenge The Day Of The Doctor for the mantle of 'Greatest Doctor Who story', this would be the one. It's a bonafide classic, and in years to come will be held with as much affection as Genesis Of The Daleks or Earthshock.