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Box of Delusions (虚妄の匣 Kyomō no Hako?) is the 3rd chapter of the 15th volume and the 55th chapter overall of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.
After Kai, Hugo, Ivo and Felix agree to meet up at the maintenance area after training, they are surprised to see Rosa up and about again so soon; after a misunderstanding, they follow her and participate in the day's training. Later, they meet up and eventually do spot movement inside the facility. After getting spotted by Kuklo, they all go towards the maintenance area and are alerted that someone is inside. They respond, only to find a familiar face waiting for them inside!
During breakfast, Kai whispers to Hugo, Ivo and Felix and confirm they are to meet later that day at the maintenance room after training. Just then, Rosa suddenly appears and surprises them all with how she is back on her feet so quickly. When she mentions how she will be training later that day, Felix expresses some worry but Rosa reveals to them Sharle told her about the sabotage. Rosa is surprised to hear that Kuklo and Cardina are working on finding the culprit, but has faith in them and Sharle.
Changing the subject, Rosa inquires if they were planning something behind her back. After Felix tries to come up with an excuse, Kai and Ivo cover for him and mention it was nothing but "guy talk." This causes Rosa to call them disgusting and storms off. Just then, the bell sounds and they hurriedly finish breakfast and go off to training.
That night, Kai, Hugo, Ivo and Felix meet up at the maintenance shed and begin taking turns watching the entrance. Kai proposes they move closer and set up an ambush but Ivo mentions they would not be able to hide from both the senior members and the culprit. However, they do agree that since Sharle is the only one there (due to Angel leaving earlier in the day), they will rush in if they have no other choice. Unknown to the trainees, Fuchs is camped out in a tree just above them and overhears their plan.
Eventually, Felix spots movement from inside the maintenance area and they move in closer. At the same time, Sharle gets a knock on the door to the maintenance room and is lured away; as Kai and Hugo look inside the window, Kuklo appears and chastises them. As Felix begins to explain, Kuklo silences them and informs them he was the one who lured Sharle away and any loud noises will spook the culprit; allowing them to join him, Kuklo leads the way to the maintenance room. Suddenly, a loud crash is heard and both Felix and Ivo run towards it. Opening the door, Felix sees someone inside; recognizing the person, he tries to attack them but is easily knocked aside. Kuklo narrows his eyes as the others notice that the person is none other than Xavi.