One by One

Koko (個々) is the 2nd chapter of the 4th volume and the 15th chapter overall of the Attack on Titan manga, written and illustrated by Hajime Isayama.

Overview
Two years succeeding the fall of Wall Maria, Armin along with Eren and Mikasa are in the fields overhearing a triggering conversation between Military Police officers. Not long from then, Armin is in boot camp getting through initiation by being intimidated by their instructor. Their instructor does the same to a few other cadets. At around sunset, the group of cadets speculate Sasha running laps as her punishment for eating a potato in front of her peers. Eren and Armin are quizzically asked by other cadets about their experience with the Colossus Titan. Jean decides to pitch in, starting a fight with Eren. As soon as the dinner bell sounds, the pair decide to call it quits. Mikasa walks past Jean, catching his eye, but after spotting her with Eren, Jean feels somewhat betrayed. Sasha finishes her running and Krista offers her some nourishment.

The next day, the cadets work on their Vertical Maneuvering Equipment skills. Unlike most, Eren finds that he is incapable of balancing.

Summary
Two years after the first attack of the Colossus Titan and three years before their graduation from the 104th Training Corps, Mikasa, Eren and Armin are working in a field. A small group of Military Police officers are walking by and discussing productivity. The officer in charge is complaining because the fields are behind schedule. His subordinate says that most of the workers were kicked out and sent to recover Wall Maria, likely killed. Armin, glaring at the soldiers, tells Eren and Mikasa that his parents were sent on that mission not for recovery, but so the government would have less mouths to feed.

A short time later, Armin is now in boot camp. The drill instructor, pacing through the ranks, singles out Armin, asking his name. After a short, intimidating speech, the instructor moves on to another trainee. Two nearby officers recall their time in boot camp, with one questioning the point of intimidation. The other answers that it is used to cultivate the necessary qualities of a soldier. As the drill instructor questions Annie, she seems unaffected, leaving one to say that she's been through her rite of passage. The instructor notices the look on a lot of the faces, and assumes it was because of the attack from the Colossus Titan. Now onto Jean, he asks why he is here. Jean answers to join the Military Police and live in the Inner District. With that, the instructor head butts Jean, causing him to fall. The instructor then adds insult to injury and says that he never gave him permission to sit. He asks Marco the same, Marco answers that he's from Jinae Town and wants to join the Military Police so that he can be of service to the King. The next up is Connie, who mistakenly uses the wrong hand in his salute. When giving him a hard time, the instructor notices Sasha, who is eating while in formation. He rushes over to her and asks what the big idea is. She's still chewing her food which causes her not to answer, so he gets louder. He asks what she is eating. She replies a steamed potato and as the situation gets more tense she breaks it in half (actually, the piece she gave to the instructor was much smaller than hers) and offers it to the instructor.

Now out of formation and at the barracks, Eren and others watch as Sasha runs laps well into her fifth hour and having to skip lunch (her reaction about having to skip lunch was much stronger than having to run "until she collapses", the latter she took quite literally). They begin to talk more about where they are from. When the conversation gets to Eren, he tell them he is from Shiganshina District, which surprises the others and causes him to constantly be asked about the Colossus Titan. As more and more questions come, Eren starts to think of his mother being killed and he gets nauseous. Marco and Connie notice and apologize for making him talk about it. Regaining his composure, Eren dismisses it and says that he can't wait to fight and kill the Titans. At hearing this, Jean accuses Eren of faking it while actually pissing his pants. Deciding to let it go, the two apoligize for the short argument. Mikasa walks by, which grabs Jean's attention. He tells her that he's never seen anyone like her and that she has pretty hair. She thanks him and walks away. Outside, she catches up with Eren; Jean sees the two together outside. Eren denies almost getting into a fight and tells Mikasa that her hair is getting too long; it could cause her to get into an accident. She asks him how long she should cut it. Seeing this, Jean wipes his hands on Connie's shirt. Connie angrily asks if his shirt looks like a tissue and what did he wipe on it. Jean replies that it was his trust in others.

Sasha, now done with her laps, is on her way to her bunk when she collapses. Krista walks nearby with a little bit of bread, causing Sasha to lunge at her, grabbing the bread. Krista says that she brought it for her and asks if she wants any water. Grateful, Sasha sees Krista as her savior. She's helped by another girl in getting Sasha to bed.

The next morning, the students start their Vertical Maneuvering Equipment training. Tying ropes to their sides, they will test their balance. Mikasa, Jean and Connie are doing well, but Eren seems to be having problems, leaving him embarrassed.