Rather than a biopic, Tarkovsky's son compiles a documentary that draws from his father's statements to narrate his artistic life. Little is shown about his private life (except his formative years), and the focus is on him as an auteur/director. Unlike most documentaries, there are no interviews or voice-over narrations. The only voice we hear (together with Arseniy Tarkovsky's poetries read by him) is Tarkovsky, as he explains his experience and his artistic concepts and his cinematographic evolution. I would say that the main focus of the documentary is Tarkovsky's spiritualism, his concept of how poetry requires spirituality, and art is a form of prayer.
While most of the material is drawn from pre-existing media, I still found a few very interesting elements: I didn't know that Tarkovsky directed Hamlet on stage with Anatoliy Solonitsin, and found it very delighting to see colour backstage recordings of the making of Andrei Rublev. The other element that Andrei A. Added, is the new recordings of some of the places where Tarkovsky lived: his Russian house, the house in Florence, etc., paired with recordings of the locations of Tarkovsky's films (the city used in Andrej Rublev, the unfinished church from Nostalghia), as well as additional frames of natural scapes shot in tarkovskian taste, such as the opening shot, used for the poster.
If you like Tarkovsky, make sure you see this!