As part of the new special episode marking the 50th anniversary of Doctor Who, the world's longest running science fiction series, show-runner Steven Moffat promised a story that would change the Doctor forever. And he delivers.

The last time a special anniversary episode of Doctor Who was screened on the show's birthday, 25 years ago, we got the Sylvester McCoy-headlined Silver Nemesis. The Day Of The Doctor is in a different class.

"This Has All The Makings Of Your Lucky Day," says Simon Brew of Den of Geek.

Here's what critics have to say about the very special The Day Of The Doctor:

Matthew Looker of IGN appreciates Moffat's attempt at keeping the essence of the show intact. "Steven Moffat chooses to spend a great proportion of time telling a spirited adventure in the style of the classic Doctor Who that made older fans fall in love with the show in the first place."

Talking about the plot, the review says: "At the heart(s) of the episode, we have a simple tale of Doctor Who baddies The Zygons – shapeshifting monsters that can mimic people (and animals) around them but otherwise look like big, ugly suckers – breaking free from oil paintings in London's National Gallery to take over the world. Meanwhile, Matt Smith's Eleventh Doctor jumps through a time fissure to team up with David Tennant's Tenth Doctor and save the day. With help from a feisty Queen Elizabeth I too, because what's Doctor Who without an important historical figure getting stuck into the action?"

Ben Lawrence of The Telegraph praises the cast members and the witty banter that fans will surely appreciate. Matt Smith and David Tennant share a fascinating camaraderie. Both actors are unique in their own different ways. "Tennant is edgy and mercurial, likely to turn on a pin. Smith is gentler, with a boyish eccentricity and other-worldly strangeness. They sparred terrifically with a fair amount of trademark humour. Smith's Doctor teased Tennant's about his 'sand shoes' and his weight. And yet they were both skilled enough to convince the viewer that they were one and the same person, both sharing a compassion, an acute intelligence and a formidable nose for danger," he says.

"John Hurt was a fantastic counterpoint to the physicality of Tennant and Smith. With one withering look, he was able to silence his younger selves. 'Am I having a mid-life crisis?' he asked with all the bewilderment of a man who, ironically for an alien, had just been confronted with his own mortality."

According to The Huffington Post review: "Day of the Doctor is the culmination of a drawn-out creative achievement on a scale as epic in the planning as in the mischievous, moving, masterful results. With a flash of Doctor future, a tantalising glimpse of Mr Capaldi's furrowed brow, and, when we thought we needed no more, a blast of delightful Doctor past, with the unmistakable Baker bellows."

The IGN review summarises the episode: "Overall, the episode faltered somewhere in the middle, but with plenty of entertaining back-and-forth between our most recent two Doctors (the ones with the sand shoes and the bow-ties), hilarious plot turns and a story that paid tribute to every element that makes Doctor Who such a brilliant show – including the real hero of the story turning out to be that often overlooked ingredient, the Doctor's companion – this was a truly triumphant anniversary."

The Day of the Doctor was simultaneously broadcast in over 70 countries worldwide, making it one of the biggest TV events of the year.

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