In 1949, at the age of eight, the concert pianist Martha Argerich made her debut in her native Buenos Aires. She was so good that president Juan Perón appointed her parents to diplomatic posts in the Argentine embassy in Vienna just so that Martha could study with Friedrich Gulda.

By 16, Martha was winning European competitions, including the Geneva International Music Competition and the Ferruccio Busoni International Competition, within three weeks of each other. At 24, she won the seventh International Chopin Piano Competition in Warsaw and became a best-selling recording artist.

She has, reputedly, an artistic temperament to match her seismic talent. Her two marriages did not end well. She cancels performances with little notice. She changes programmes.

This restrained, fond, and often awe-struck portrait of Argerich by her youngest daughter, Stéphanie, is a lovely, heartfelt thing that takes us all around the world: they’re a very international family.

This is not hagiography, but nor is it warts-and-all confessional. Family secrets – notably the teenage sister who suddenly turned up when Stéphanie was four – are explored. There are interesting questions about maternal identity, and how it can both clash and dovetail with career and genius.

Some questions do not have easy or satisfactory answers, especially when asked of Martha, who, in common with many great artists, simply cannot find the words to convey her intimacy with Chopin or with the instrument she started to play at the age of three: “I don’t know how to explain my relationship with the piano,” she shrugs.

The director allows her mother to answer in an oblique way, as she often does. But the gaps in our knowledge only serve to underline her humanity.