The film documents a project in Halberstadt, Germany, to perform John Cage's composition 'As Slow As Possible'. The performance, on a church organ, began in 2001 and will last 639 years. The note changes, once every couple of years, have turned into 'mini concerts' in their own right. (The first year and a half, by the way, was a silent pause.)
This, ehm, lengthy and unavoidably philosophical concert is a consequence of the simple fact that Cage never specified how slow his piece was to be played. It is the kind of theoretical exploration Cage became famous for. His oeuvre also includes the composition '4'33"', consisting of four minutes and thirty three seconds of silence. (For a great performance, see this video.)
In this case, the sheer length of the performance - assuming it will be completed by future generations - is so mind-boggling it can only be compared with the building of cathedrals or pyramids. Except there will be no monument - it only exists as long as it lasts, the ultimate testament to the fleeting nature of sound, and of time itself...
If you happen to be in Halberstadt (near Leipzig) during your lifetime, don't miss this concert!