Matt Hurt’s review published on Letterboxd:
A pretty dry look at Mifune's career and his relationship with Kurosawa, in particular. I really wish it had dived deeper into Mifune's life and personality. Unfortunately it doesn't spend much time on the topics it covers. Instead, it's pretty much an overview of the subject of Mifune, with bits of Japanese history and Japanese cinema thrown in as well.
Keanu Reeves narrates the doc but doesn't put any interesting spin or personality into the narration. It's a pretty straightforward, almost dull, narration.
Regardless, it's still an interesting enough look into Mifune, Kurosawa, and Japanese film history. If the subject interests you, you probably won't be that disappointed.