According to this, the BBC is searching for Bob Dylan's acting debut, which they apparently lost:
The BBC has launched a worldwide search for a lost 1960s television drama which featured the acting debut of Bob Dylan, the musical hero of a generation. Producers on Arena, the corporation's flagship arts programme, hope that one of the star's huge army of fans will have a copy of The Madhouse on Castle Street, which was aired just once, in 1963.
...The programme, which features Dylan's first televised performance of Blowin' in the Wind is now classified as missing by the BBC. Arena wants to include the film, which also starred David Warner, in a series to celebrate Dylan's life and work. The series, to be broadcast this year, will also feature a two-part documentary by Martin Scorsese, the American film director who is a fan.
Anthony Wall, Arena's editor, told The Sunday Telegraph: "The programme is not in any archive and we are hoping that someone out there will have a copy. There is a huge market for Dylan memorabilia so we may stand a chance. "All we have is the script and part of the soundtrack. For us the play is the Holy Grail of missing film."
...Mr Wall has spoken to three people who have vivid memories of watching the film. They describe the star's musical performance, which also includes a rendition of Ballad of the Gliding Swan, as "life changing". Mr Wall said: "The drama itself was very Pinteresque, like many programmes popular at the time. Millions of people would tune in and then wonder what it was all about at the bus stop the next day.
"Dylan, perhaps not surprisingly, played a folk singer who spent most of his time sat on a flight of stairs. . . and commented on the ongoing action through songs and dialogue. "I think Dylan expected to have a much larger part but I think he could not handle the dialogue. It is my understanding that this was the very first time he sang Blowin' in the Wind on television."
I don't know if this qualifies as a Holy Grail. A Dead Sea Scrolls maybe, but not a Holy Grail.