The time is early in the 17th century, 16 years after the death of Toyotomi Hideyoshi, who brought a long period of feudal wars to an end and unified Japan under his sole power. In large part because Hideyoshi’s heir Hideyori was still a small child at the time of his father’s death, the intervening years have seen control of the country pass to Tokugawa Ieyasu. Against this backdrop, the story follows the lives and escapades of several young ninja as they become caught up in events surrounding the Siege of Osaka Castle?the campaign to defeat the last challenge to Tokugawa power now being mounted by the grown-of-age Hideyori.
After undergoing rigorous and exacting training from childhood to acquire the necessary stealth and fighting techniques, the ninja of this period were hired by warring parties to conduct espionage, sabotage, and assassinations. Young Pūtarō went through the training of the Iga ninja clan, but together with his pal Kuroyumi was sent packing after failing the test for admission to the ranks of full-fledged ninja. At a loss, the two made their way to Kyoto, where they went their separate ways. Having exhausted his financial resources, Pūtarō has been reduced to living in a hovel on the edge of town and eking out an existence as a day laborer. Then, about a year and a half after the day they parted, Kuroyumi shows up carrying a large collection of gourds and asks Pūtarō to help deliver them to a gourd shop named Hyōroku in the city. There, the shopkeeper asks Pūtarō to grow more for him. Although he has had difficulty getting motivated about anything else of late, Pūtarō takes avidly to cultivating gourds and has soon produced some very fine specimens, which in due course he takes to Hyōroku. This time the shopkeeper asks him to make a delivery of gourds from the shop to the Kyoto residence of Hideyoshi’s widow Nene. Nene tells Pūtarō that a certain high personage of about his age?she is in fact speaking of Hideyori?needs a security detail for sightseeing in Kyoto. Pūtarō accepts the assignment together with Kuroyumi and a third ninja their age named Tokoyo.
With the grown Hideyori becoming an increasingly troublesome thorn in his side, Ieyasu has been looking for an excuse to lay siege to his castle in Osaka. When the first battle between Tokugawa and Toyotomi forces known as the Winter Campaign begins, Pūtarō, in spite of having been sent packing earlier, receives orders from the Iga ninja boss Uneme to join in the clan’s efforts on behalf of the Tokugawa. To a ninja, the boss’s orders are absolute. But while maintaining an appearance of faithful obedience, Pūtarō and his peers harbor different feelings, which they are careful to keep to themselves. Kuroyumi continues to hustle for money. The fair-faced Tokoyo assumes a female disguise and is sent into Osaka Castle as a lady in Hideyori’s service. Pūtarō is followed around by Semizaemon, a character who seems to be keeping tabs on him. The female ninja Momoichi disguises herself as a courtesan and spreads disparaging rumors about Toyotomi partisans. Amid these many intrigues, the true desire of Pūtarō and his peers is to be freed from the shackles of their ninja oath.
Surrounded by constant scheming and unable to tell friend from foe, the young ninja become entangled willy-nilly in the mounting conflict, and they find themselves increasingly sympathizing with Hideyori. When the final battle known as the Summer Campaign begins, Nene asks Pūtarō to deliver a dagger to Hideyori, and he smuggles himself into Osaka Castle together with Kuroyumi, who is going to bid farewell to Hideyori and return to his native Macao, and Semizaemon, who is under orders by Uneme to kill Tokoyo. The castle is on the verge of falling. When Pūtarō is finally able to see Hideyori, Hideyori asks him to smuggle his baby daughter out of the castle. With the help of Kuroyumi, Tokoyo, and Semizaemon?who have all decided on their own to aid the Toyotomi cause in whatever way they can?Pūtarō attempts to break through the 100,000-strong Tokugawa encirclement of the castle with the baby in his arms. But the odds are far too heavy, and only Kuroyumi manages to escape. After Tokoyo and Semizaemon fall, Pūtarō, too, is mortally wounded and on the verge of death, when Momoichi appears. Pūtarō entrusts the baby to her and then breathes his last.