"I just want the girl I love to be happy."

The members of the Shinsengumi all share a close companion: her name is Okita Mitsuba. She's the older sister of Sougo and has been a close friend of Kondou and Hijikata's even before the Shinsengumi was formed. She's kind, gentle, has an affinity for spicy foods and crackers, and is the friend that the three of them left behind when departed for Edo to form the Shinsengumi. Mitsuba has watched over Okita as his guardian since he was very young, thus his constant worry over her well-being and happiness. She is also quite ill, and has been suffering from lung disease for quite some time.

Okita's sister makes her first apperance in Gintama during a visit to Edo to meet with her fiancee's family, and stops at the Shinsengumi Headquarters to announce her upcoming marriage. The men are thrilled to hear of the news, and Kondou reassures Mitsuba that the Shinsengumi will support and attend her upcoming wedding. Kondou gives Okita the day off and asks him to spend time with his sister and show her the sights of the capital.

"He's a guy I never liked from the beginning."

Okita sees his sister's marriage as an opportunity for her to find happiness in a life that's been full of sickness and sad goodbyes. However, at the same time that Okita (and Gintoki, whom he introduces to Mitsuba as his "friend") and his sister are enjoying each other's company, Hijikata and Yamazaki are investigating a merchant associated with the Jouishishi--the same merchant whom Mitsuba is engaged to. His name is Kuraba Toma, and he runs a trading company called Tenkaiya which the Shinsengumi have been tailing closely. Later that night, Mitsuba returns to her fiancee's mansion with her two companions and before saying goodbye, asks Okita about Hijikata. The mood quickly darkens as Okita tells his sister he won't allow her to see him, and that he had left that morning on a job without saying anything.

At that moment, Hijikata and Yamazaki also arrive at the mansion and upon seeing Hijikata's face, Mitsuba begins coughing and passes out. The Shinsengumi members and Gintoki take her inside to rest, where they meet with Mitsuba's fiancee, Kuraba. However, Okita finds himself incredibly upset with the fact that Hijikata dared to show his face in front of his sister. Hijikata and Yamazaki take their leave without saying much, and at this point we're granted a small amount of flashbacks from Mitsuba's past that begin to illuminate why the two Shinsengumi members who are constantly at each other's necks. The first scene shows a much younger and crying Okita, who doesn't want to go to practice at the dojo because of the new guy who "pisses him off". Okita claims that he's cocky and talks to him like an equal, despite his junior status, and also mentions that Kondou no longer pays attention to him since the newcomer's arrival. Another scene shows the four of them together eating lunch, where Kondou is yelling at both Mitsuba and Hijikata who are over-seasoning their dishes with their favorite condiments (pepper and mayonnaise, respectively). While the three seem to be enjoying each other, Okita looks on jealously.

Later that day, Okita and Hijikata meet up at the dojo, where they have a conversation regarding Tenkaiya and the Shinsengumi's job to apprehend the criminal. While Hijikata seems to remain immovable on his decision to arrest Mitsuba's husband, Okita's main concern is with his sister's happiness.

Okita: She doesn't have much longer. I'm told she doesn't have much longer to live. I can't stand it. Since I was little, she took care of me, ignoring her own needs, and even... But now when she's about to grasp happiness at last. At least before she dies, I want her to experience happiness like other people. I'm not saying to let him go. Just for a little while... Hijikata-san, Sister stayed single all this time because you were...
Hijikata: The deal goes down tomorrow night. Make sure your sword's in good shape.
Okita: I don't like him. He's always like that. He just happens to show up and swipe everything I hold dear.

"How many times will he stand in the way of my sister's happiness?"

While Okita's hatred for the vice commander often comes off as childish and purely out of jealousy, it becomes clearer in another flashback that his dislike is also a product of the fact that Hijikata had broken his sister's heart once before. For someone as concerned with his sister's happiness as he is, to be a witness of her confession to Hijikata was probably quite jarring for him.

Mitsuba: Is it true you're all going to Edo to find success?
Hijikata: Where'd you hear that?
Mitsuba: So-chan... he looked so happy this morning.
Hijikata: That idiot.
Mitsuba: Take me with you. I'm... after all, like a parent to So-chan. If I'm not there, that child... And also because... I... Everyone... I want to stay with you, Toshirou-san.
Hijikata: Whatever. What you do is up to you.

However, despite his anger towards Hijikata, Okita knows in the back of his mind that this sort of answer was the only option. Mitsuba was constantly treated coldly by Hijikata, but regardless of this fact she always waited for him to come back to her. Hijikata's reasonings for treating Mitsuba in such a fashion were simply for her own good: a man who constantly faces death couldn't possibly risk taking care of her. He ultimately dismissed her in this way so that she could move on and live a happy life with a more well suited man. Okita's frustration stems from the fact that when she finally moved on and found someone else, he came back into the scene.

With Mitsuba now in the hospital in her worsening condition, Yamazaki remains aware of his relationship with Mitsuba and begs Hijikata to forget about work and stay by her side. However, the vice commander quickly retaliates with refusal and claims that though he may appear cold and heartless, her husband is out trying to cut a deal while his wife is dying. Hijikata makes sure that nobody else in the Shinsengumi knows about the ordeal, and goes to pursue Kuraba and Tenkaiya on his own.

"Work comes first, as usual, it seems."

Yamazaki, worried about the vice commander's safety, meets up with Okita and Kondou at the hospital in a state of panic to let the Shinsengumi know what Hijikata has decided to undertake on his wn. He reports to Kondou that Hijikata went to do the job by himself, because if anyone found out that Okita's relative was related to Tenkaiya, Okita would lose his position as captain. The young commander's growing frustration begins to show, and he begins angrily speaking to Kondou about how he doesn't want to be indebted to Hijikata, and that he constantly felt a gap between himself and the rest of the Shinsengumi. In the heat of the moment, Kondou punches Okita and speaks to him in an uncharacteristically stern manner. He explains to Okita that a friend is someone who is willing to punch his comrade back into line when they've gone astray.

After a silent apology to the dying Mitsuba, the Shinsengumi gather up their forces to join Hijikata. While the rest of the Shinsengumi are making their way to the scene, Hijikata finally faces Kuraba at swordpoint and the merchant begins to explain his reasonings. By marrying Mitsuba, he planned on getting the backing of the Shinsengumi so that he could run his business without any restrictions. He had figured that being related to his sister would make Okita easy to control, but his plans had fallen through when the doctor told Kuraba that Mitsuba didn't have much longer to live. He loved Mitsuba simply as a tool for running his business more succesfully.

HIJIKATA: You and I are alike. I've also done countless horrible acts. And to top it off, I'm trying to kill the husband of a woman who's dying. yeah, that's pretty cruel.
KURABA: No wonder they call you the the demonic vice commander. I think you and I will get along well.
HIJIKATA: That's really... an exaggeration. I just want the girl I love to be happy. A life like that is impossible for a guy like me who lives by the sword. I just wanted her to marry a normal guy somewhere, have kids like everyone else and live a normal life. That's all.

Together as comrades, and with a pass off of spicy crackers from Gintoki to Hijikata, Okita and Hijikata are able to apprehend and rid of Kuraba together. The Shinsengumi make it back to the hospital just in time to catch Mitsuba's last few breaths. With her last words, Mitsuba reassures her younger brother that she was never unhappy, but quite the opposite. She was glad to have such wonderful people in her life and watch those people pursue the path they had chosen. Hijikata sits not by her beside, but on the rooftop of the hospital, fighting back tears and eating a bag of Mitsuba's favorite spicy chips.