This is a story of dirty politics and betrayal. The Iga ninja leader Hattori, Hanzou III is once again disappointed with politicians, including the one he's forced to work with.
Peasants revolt. The government blames the Yashu Karasuyama samurai clan and orders the disolving of the clan and the vacating of Karasuyama castle. A smaller and distant clan, the Matasushiro Sanada, is ordered to take the castle within 20 days. The Sanada leader passes the job onto a branch clan the Hirakobo. Sure that the larger Karasuyama won't leave their castle without a fight, the Hirakobo leader's guardian (the actual head of the clan being only 11 years old) tries to hire extra samurai.
The Kouga spy Okou tells Hattori Hanso that she was raised in a samurai family of Hirakobo and begs his help. Was she telling the truth or was this just a plot to get Hattari Hanzou killed by sending him to fight a lossing battle?
Hanzou asks Hoshina why a large, closer clan, such as Hoshina's own, wasn't assigned to take the castle, but doesn't get a satisfactory answer, Hoshina is after all a politician. So while setting out for the castle with his other ninja, Hanzou leaves his right hand man, Hyoroku, in Edo to find out what political machinations are behind this seemingly irrational act of the government. Okou has a chance to hurt Hanzou by letting his right hand die from a wound received when stealing a secret document, but doesn't take it, instead she helps him. Hyoroku gives the secret document to Okou to pass on, but to whom does she pass it, the Iga or the Kouga?
The highway is a devil because a group of ninja ambush the samurai and the Iga who travel with them. I'd name the ninja clan but that would be too much of a spoiler. Naturally Hattori, Hanzou defeats the other ninja leader. Of course the government official behind the evil scheme to wipe out two innocent clans goes unpunished, much to idealist Hattori, Hanzou's disgust.