The hero of this documentary is a boyishly bespectacled fellow named Stefan Knupfer, who with his magic wand could be the Viennese cousin of Harry Potter. In fact, he's a tuner for Steinway & Sons, and his wand is a thin metal jack for tweaking the strings of their individually numbered pianos.
Perfect pitch, inexhaustible patience and a natural dose of Teutonic obsessiveness are the talents Stefan exercises to ensure that the world's greatest pianists – Lang Lang, Alfred Brendel, Pierre-Laurent Aimard – are playing an instrument as close to perfection as it can get. Problem with the hammerhead on the D key? He's your man to fix it, and a myriad of similarly minute calibrations. The film rather labours the point during the studio recording of a Bach suite – engineers twiddling the sound controls is not of unlimited interest – but as a record of unseen (though not unheard) devotion to excellence this is stirring stuff.