Nine missing episodes of 1960s Doctor Who have been found at a TV station in Nigeria, including most of the classic story The Web of Fear.
The black and white story sees Patrick Troughton's second Doctor battle robot yeti in the London Underground.
Also recovered is a complete version of Troughton's six-part story The Enemy of the World.
It is thought to be the largest haul of missing episodes recovered in the last three decades.
"It's thrilling," said Mark Gatiss, an actor and writer for the 21st Century incarnation of Doctor Who.
"Every single avenue seemed to have been exhausted, every now and then something turns up - but to have two virtually complete stories out of the blue is absolutely incredible."
The BBC destroyed many of the sci-fi drama's original transmission tapes in the 1960s and 1970s.
However, many episodes were transferred on to film for sale to foreign broadcasters. It is often these prints found in other countries that are the source of retrieved episodes.
In this case, 11 Doctor Who episodes were discovered, nine of which were missing, in the Nigerian city of Jos.
The find was made by Philip Morris, director of a company called Television International Enterprises Archive.
Mr Morris said: "The tapes had been left gathering dust in a storeroom at a television relay station in Nigeria. I remember wiping the dust off the masking tape on the canisters and my heart missed a beat as I saw the words, Doctor Who. When I read the story code I realised I'd found something pretty special."
He said it had been a "lucky" find given the high temperatures in the African country. "Fortunately they had been kept in the optimum condition."
Only episode three of The Enemy of the World already existed in the BBC archive. The Nigerian discovery of episodes one, two, four, five and six complete the story.
Episode one of fan favourite The Web of Fear existed, with the rest thought lost forever. Now episodes two, four, five and six have been recovered.
Episode three is still missing, but has been reconstructed from stills to enable restored versions of both stories to be made available for sale via download on Friday.
The latest find means that the number of missing episodes of Doctor Who has dropped from 106 to 97.
One episode from each story - both last seen in 1968 - were shown at a special event in London on Thursday by BBC Worldwide, the BBC's commercial arm.
Among the guests were actors Frazer Hines and Deborah Watling, who played Troughton's Tardis companions Jamie McCrimmon and Victoria Waterfield.
Episode one of The Enemy of the World is a James Bond-style thriller complete with an exploding helicopter, a hovercraft, gun-toting henchmen and a foreign-accented villain, Salamander (also played by Troughton).
The story opens with the Tardis arriving on an Australian beach where the Doctor strips to his long johns and goes for a dip in the sea.
The Web of Fear is a claustrophobic tale that sees the Doctor battle his old foe, the Great Intelligence, and the yeti in the tunnels of the London tube system.
"It's the quintessential Doctor Who story," said Gatiss. "It has the return of the Abominable Snowmen in an iconic location."
He said it showed Troughton "at the height of his powers".
Frazer Hines recalled that the underground station sets had been so realistic that London Transport accused the BBC of filming at a tube station in secret.
The story also featured an appearance by Deborah Watling's real-life father Jack, reprising his role as Professor Travers.
Recalling Troughton's "wonderful sense of humour" on set, Watling said: "We all got on so well, we were like a family and Pat was always to me like another dad or an uncle. We had a chemistry and I think it showed."
How did she feel when she heard about the recovery of the lost episodes? "I couldn't quite believe it. There had been hoaxes before. I thought it was just another hoax."
Her only other complete story in the archive had been The Tomb of the Cybermen, all four instalments of which were discovered in Hong Kong in 1991.
Hines said: "This now gives me hope that more stories of Patrick's will come out of the woodwork."
The latest find comes as Doctor Who celebrates its 50th birthday. A special episode featuring the current Doctor, Matt Smith, and his predecessor, David Tennant, will be shown on the programme's anniversary on 23 November.