Smith, the youngest actor ever to play the Time Lord, bows out in spectacular style in the Christmas Day episode
Smith, the youngest ever actor to play the Time Lord, bows out in spectacular style, confronted by a "greatest hits" of the doctor's enemies in a snow-covered faraway town called Christmas.
But the show, the first full appearance by The Thick of It star Capaldi in the role, which celebrated its 50th birthday last month, is also a sombre affair, reflecting on mortality and the passing of time.
Showrunner Steven Moffat asked the audience at London's British Film Institute, on London's South Bank, not to reveal too much about the episode, which brings an end to Smith's turn as the doctor after four years.
Guests included BBC director general Tony Hall, culture secretary Maria Miller and BBC presenter Professor Brian Cox, who said it was "magnificent, complex and emotional, beautifully acted. It was impressive that it drew on the whole history of Matt Smith as the doctor".
The festive special, called The Time of the Doctor, begins with every viewers' Christmas nightmare – daleks, cybermen and a turkey which takes 300 years to cook. It also features some unlikely Who nudity (although not too much for a teatime audience).
Hall described Moffat, co-creator of BBC1's Sherlock, another centrepiece of BBC1's holiday schedule, as an "extraordinary man, a real hero of the BBC".
Moffat said it was too soon to talk about how Capaldi's interpretation of the doctor would take shape. The only scenes he has filmed so far are the ones which conclude the Christmas special.
Capaldi began his first read-throughs in the role on Tuesday, with the next series due to begin on BBC1 next autumn.
Moffat said Smith's swansong as the doctor was a "huge, heartbreaking moment, and that's what we like to give you at Christmas". "His very last moment in the Tardis is a solemn moment, as Christmas always is," he joked.
The doctor's regeneration into Capaldi is as spectacular as the scenes which saw former incumbent David Tennant transform into Smith four years ago.
It comes at the climax of the doctor's battle against the massed forces of the universe's deadliest species, drawn together by a mysterious signal echoing throughout the universe, filling them with dread.
"Everything ends," the doctor tells his companion and best friend, Clara, played by Jenna Coleman. "Except you," she tells him.
"It's a moment of huge sadness but it doesn't end there," said Moffat. "It's an insane emotional reversal, and that's what Doctor Who is made of."
Moffat said Smith's tenure had been "heartfelt, brilliant, an elegant shambles. He caught the age of the doctor beautifully".
The episode is likely to be one of the most popular shows on Christmas Day, although it faces stiff competition from the likes of BBC1's Call The Midwife, ITV's Downton Abbey and the traditional ratings winners, the soaps.
Comedian Frank Skinner, who was also at the screening, said: "I loved it. It addressed several issues that have been lying around the Doctor Who world for years. I am a big Matt Smith fan and I thought I had seen how brilliant he is. In this he was even more brilliant, the perfect swansong."
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