Doctor Who fans are getting a fresh opportunity to travel back in time with the discovery of two missing episodes from the long-running BBC series.
The 1965 and 1967 episodes star William Hartnell and Patrick Troughton, the first two actors to play The Doctor.
They are among more than 100 instalments which were not retained by the BBC.
The announcement was made on Sunday at Missing Believed Wiped, an event held at the British Film Institute (BFI).
The BFI, based in London, has been working with broadcasters and film collectors to recover missing recordings of many different television series.
Television programmes only began to be routinely recorded in the late 1950s. Previously, they were broadcast live.
Even when it became possible to make recordings, video tape was so expensive that most transmission tapes were wiped, so they could be re-used.
In the 1970s, the BBC commissioned a report by the historian Lord Briggs. His findings prompted efforts to improve the archiving of television programmes.
Although original transmission tapes did not survive, programmes were transferred on to film for broadcasters abroad and it is these prints that the BFI sometimes recover.
Doctor Who was sold all over the world in the 1960s.
The latest two episodes to be found were in the private collection of a retired broadcast engineer, Terry Burnett, who bought them at a school fete in Hampshire in the 1980s.
Mr Burnett had no idea the BBC did not have the recordings - it was only when he mentioned them casually in conversation to Ralph Montagu, head of heritage at Radio Times, that their significance was recognised.
Airlock is the third episode of a four-part story called Galaxy Four, transmitted in the autumn of 1965.
Hartnell is accompanied by Peter Purves, later a presenter on the children's series Blue Peter, and Maureen O'Brien, now better known as a novelist.
The plot involves the desperate attempts by the Drahvins, a race of cloned females, to escape a planet which is about to explode. The story also marks the only appearance in the series of tiny robots known as the Chumblies.
The other re-discovered episode is the second part of The Underwater Menace, from early 1967.
It sees a mad scientist attempting to restore Atlantis by draining the ocean into the Earth's core.
Patrick Troughton, in only his third appearance as The Doctor, was apparently unimpressed with the production.
In a recent biography, his son Michael says he argued with the director Julia Smith - who went on to become the original producer of EastEnders - complaining bitterly about the "ridiculous costumes and make-up of the fish people".
Innes Lloyd, who produced The Underwater Menace later admitted, "it did look like something from a '50s American 'B' movie".
The find makes only a modest dent in the number of missing episodes, with 106 instalments broadcast between 1964 and 1969 still being sought.
Among other items recovered and shown at Sunday's event were Emergency Ward 9, a play by Dennis Potter, and a comedy sketch featuring Peter Cook and Dudley Moore.