In a world in which having large breasts means everything, the successor of the powerful Manyu family, Chifusa Manyu, defects and steals the Manyu Scroll of Secrets (which contains all the Manyu secret techniques for breast enlargement), but is wounded during her escape. She is found by a woman who was recently attacked by Kagefusa, a Manyu assassin and Chifusa's sister, resulting in her breasts becoming small and effectively making her poor. The woman becomes jealous of Chifusa and attempts to sever her breasts in a fit of rage, but Chifusa manages to calm her down. As Chifusa is about to leave, she is confronted by Kagefusa, who had also captured Chifusa's comrade, Kaede, and severed her breasts. As Chifusa stands against Kagefusa, she uses a special technique which steals Kagefusa's breasts and adds them to her own. As Kagefusa is forced to retreat, Kaede decides to join Chifusa on her journey.

Running low on money, Chifusa and Kaede earn some money by utilising Chifusa's assets. They go to an inn run by a well-endowed landlady. However, this landlady turns out to be a Manyu assassin who drugs both Chifusa and Kaede and then hypnotizes them with her breasts, intending to turn them into her lesbian love slaves. However, before she can finish taking control of their minds, the hypnotized Kaede pounces at the landlady's breasts, releasing Chifusa from the mind control effect and giving her the chance to absorb the landlady's breasts. As they move on, Chifusa comes to realize about her ability to alter a woman's breasts using the Breast Flow technique, and vows to learn more about it so she can return Kaede's breasts to normal.

Having not eaten for days, Chifusa and Kaede get a job at a maid bar, but soon get fired due to Chifusa's short temper around the rowdy customers. Collapsing due to hunger, they are rescued by a man whose daughter was stricken by a mysterious breast disappearance. The man asks Chifusa to take his daughter's place in a contest against a neighboring village to please Hatomune Mie, a rowdy landlord. The opposing team sends a Manyu assassin to attack Chifusa, but she manages to beat him and wins the contest the next morning. Although Hatomune later hears word about Chifusa's true identity, he allows her to go, having been mesmerized by her breasts. After she leaves, Hatomune vows to find Chifusa again and get to touch her breasts.

With Chifusa and Kaede coming up short on a food bill, they are forced to become divers by the restaurant's owner, Oiso. As Oiso laments how her large breasts keep her from diving properly, her assistant, Mizuki, is attacked by a large octopus which has been causing trouble for the village for years. The next day as everyone hunts for the octopus, Oiso is captured while saving Kaede from the octopus, so Chifusa dives in after her. With the octopus using Oiso as a shield, Chifusa uses the Breast Flow technique to attack the octopus without hurting Oiso, absorbing her breasts in the process. As the octopus attacks Chifusa before she can come up for air, Oiso uses her newfound freedom to kill the octopus, giving Chifusa the credit as thanks.

Chifusa and Kaede save a woman called Osuzu from four "monsters" that assaulted her and tried to molest her breasts. At Osuzu's village, both heroines meet a man named Yasuke, who lost his massively-breasted wife two years prior and struggles with different jobs to maintain his four kids, and whom Osuzu is hopelessly in love with. In order to help Osuzu, Chifusa and Kaede convince Osuzu's grandfather to organize a hunt for the monsters, leaving Yasuke to take care of Osuzu; ultimately Osuzu manages to charm Yasuke and they begin dating. During the hunt, Chifusa and Kaede stumble upon the monsters and after a brief fight they discover that they were just Yasuke's four children, who played with their captured women's breasts because they missed their deceased mother's bosom. Chifusa strikes a deal with them and makes them promise not to assault any more women...after she lets them play with her own breasts for an entire afternoon.

After an encounter with some guards, Chifusa ends up absorbing several pairs of breasts, which make her own tits enormous and massively heavy, limiting her mobility. She and Kaede travel to a temple where a nun named Jifuni theorises that the reason for Chifusa absorbing breasts is because of her own selfish desire for her mother's comfort. Chifusa is put on a training course to cleanse and rebuild a pile of rocks that form a nipple-like fountain in order to purify her breasts. However, just before she completes the fountain, she is approached by Kagefusa, who steals the Manyu scroll only to find the Chichinagare technique cannot be learned, as it was passed down from her mother's bloodline. Noticing Kagefusa's sorrow for having small breasts, Chifusa gives her the opportunity to cut hers, but she cannot bring herself to do so because of her sister's unselfishness and because of the sheer beauty of her breasts.

Chifusa and Kaede run into Lord Hatomune Mie, who has escaped from his palace and disguised himself as a commoner to live a simple life in a village, where he has become a popular citizen. After receiving a tour of the town from Mie, Chifusa and Kaede depart but are soon stopped by a Manyu swordsman who attacks them and kidnaps Chifusa, taking her to the palace of a local lord. In the palace, the Manyu swordsman takes the Manyu scroll from Chifusa and begins to use the different techniques on her to test them out before returning her to the Manyu. Kaede asks Mie for help in rescuing Chifusa and the two break into the local lord's mansion, where Mie defeats dozens of the lord's soldiers and finally releases Chifusa from the dungeon where she is locked away. While Mie finishes off the local lord, Chifusa confronts the Manyu swordsman and kills him. After all three of them escape to safety, Chifusa rewards Mie by allowing him to play with her breasts, which Mie surprisingly refuses.

While traveling through the forest, Kaede and Chifusa save a painter called Sakuji Itamune from a trio of Manyu guards who wanted to kill him because of the content of his art. When Sakuji sees Kaede, he immediately asks her to model for his next painting. When Chifusa tells Kaede she shouldn't because it would slow down their progress, Kaede mistakes Chifusa's motivation and believes she doesn't want her to model because she is jealous, prompting her to abandon Chifusa. When they are about to start painting, Sakuji reveals he is a nude painter who only paints flat-chested models (which is why the three Manyu guards had attacked him) which shocks Kaede and makes her escape; though as soon as she leaves she is attacked by the three Manyu guards whom they had run into earlier.

Chifusa and Kaede rest for the night in an abandoned shack after Chifusa comes down with a fever. While they are resting, Kaede mentions that Chifusa had only come down with a fever like that eight years prior (in an incident she does not recall which ended with Kaede being assigned as Chifusa's attendant). Chifusa begins to recall what happened what day while at the same time, in Muneyuki's house, Ouka does the same. It turns out that Chifusa had accidentally used the Breast Flow technique before during a duel she had with Ouka eight years prior and had accidentally stolen her breasts. After recovering, Kaede and Chifusa continue their journey, concluding that Chifusa has yet another set of breasts to return after she masters the Breast Flow technique.

Chifusa and Kaede arrive to the Munemori village, hometown to Chifusa's mother, which is now in ruin. There they reunite with Chifusa's grandmother and Chifusa's aunt, Tsuhuya Munemori, and her baby daughter, Hazuki. Tsuhuya agrees to teach Chifusa how to master the Breast Flow technique. At the same time, Kagefusa and Kokage take Ouka to Edo village, where they have a day of fun and a night of drinking, during which Kagefusa and Ouka discuss the latter's feelings for Muneyuki until they are interrupted by a courier with urgent news. The following day, before the training begins, Chifusa's grandmother tells Chifusa the story of how her mother died, revealing that Chifusa's mother killed herself so the Manyu wouldn't learn how to use the Breast Flow technique for their corrupt ways. Nonetheless, Chifusa vows to learn the Breast Flow technique in order to undo the Manyu's unjust regime.

Muneyuki and Ouka lead a batallion of four Manyu assassins to capture Chifusa and Kaede. The two, along with Tsuhuya Munemori, face off agaisnt the assassins, defeating one of them before being outmatched. However, they are saved by the timely arrival of Kagefusa and Kokage, who join in on the fight. When the fight begins to get too rough, Chifusa's grandmother tells her to take Hazuki away with her to protect the last child of the Munemori bloodline; though initially hesitant, Chifusa and Kaede agree and flee with Hazuki. However, they are cornered by Ouka, who wishes to settle her old score with Chifusa as well as defeat her to gain status among the Manyu. Ouka initially has the upper hand in the fight, but halfway through it, Chifusa manages to unlock the Breast Flow by remembering how she had earlier attained a mother's heart...by breastfeeding Hazuki, which allowed her to have a motherly bond with one of her kin.