This is a 1982 one-off special that takes the legendary swordsman and strips him down to a kid learning to stop swinging wildly and start thinking. The entire thing moves at the pace of a duel: no wasted moments, no montage shortcuts. By the end you understand why Musashi mattered, not because the show tells you, but because you watched a stubborn child become someone dangerous through discipline alone.
Shounen Miyamoto Musashi: Wanpaku Nitouryuu is a 1982 drama special from Toei Animation.