K-20 steals an experimental Teslacoil and is chasing a Pieter Bruegel reproduction because it will lead him to a way to use Tesla's technology to destroy the industrialised world. I'm going to leave the plot there in case you want to check it out, which you really should. K-20 himself is framed as a full-on, V for Vendetta graphic novel anarchist, wanting to destroy the established order and create a populist utopia in its place. Much like Moore's England, the alternate Japan presented in K-20 does not want an anarchist uprising, just a more equitable society and the elimination of of poverty, so Heikichi, Akechi and Yoko have to team up and race K-20 to find the secret Tesla device and save the world. Throughout the film, Heikichi trains himself in parkour, martial arts, disguise and security, picking up gadgets from a former circus friend and developing something of a costume. By the close of the film, Heikichi is a full-fledged costumed adventurer, ready for everything the traditional Japanese hegemony can throw at him.
I've got an article brewing about how Japanese writers use typically Western (especially American) genres to explore anti-Japanese ideas, with superheroes repeatedly coming up in this literary resistance. K-20 (the film) rallies strongly against the kind of WWII-era Japanese values of society above self or happiness that films like Space Battleship Yamato or Godzilla: Final Wars (both genre cinema in their own rights) glorify as the pinnacle of citizenship. The key to Heikichi's eventual victory over K-20 isn't sacrifice, but making decisions that benefit himself at the cost of society. We're pretty used to that concept here in the West, especially when it applies to a white, cis-male, heterosexual protagonist, but in Japan, it's still something of a revolutionary concept. K-20 is a hell of a good watch; fun, fast-paced and funny when it needs to be. Check it out if you can while I look for a suitable miniature to get him on the table.