Chapter 171: Undercover Action

Undercover Action (隠密行動, Onmitsu Kōdō) is the 171st chapter of the Kekkaishi Manga written and illustrated by Yellow Tanabe.

Summary
On the school roof, Yoshimori and Sen discuss the butterflies. Yoshimori assumes they multiplied, which is problematic because they are so weak that it is difficult to detect them. Meanwhile, Tokine asks Shu to destroy any additional butterflies he comes across without drawing attention.

At home, Tokiko Yukimura senses a disturbance at Karasumori.

Yurina Kanda, Kyoko, and Ayano discuss Yoshimori leaving class early, as well as what they think of Sen. Yurina is distracted by butterflies in the classroom, but realizes they aren't normal when neither of her friends can see them.

Yoshimori and Sen find more butterflies as they move through the school. Yoshimori leaves his Shikigami to deal with the butterflies outside, and asks for Sen's help in getting rid of the ones inside without attracting notice. Together they destroy the butterflies they come across in the middle school, with Shu (who has now destroyed one hundred) and Tokine doing the same in the high school. Tokine is frustrated as more butterflies seem to appear as soon as she clears an area. She is approached by her friend Madoka, whose face is inexplicably covered in butterflies (Tokine's resulting shriek scares most of them off). Madoka explains she came to check on Tokine, who claimed she wasn't feeling well as a way to get out of class. Tokine says she'll probably miss the next class as well and hurries off.

Yoshimori and Sen begin arguing when Yoshimori accidentally forms a Kekkai around two butterflies and part of a person. Yoshimori suddenly sees a Shikigami bird bearing his grandfather's features, which signals them up to the roof where Shigemori Sumimura is waiting. There, Yoshimori and Shigemori begin arguing over Shigemori's insistence on wearing his Kekkaishi attire in broad daylight. Sen is intrigued by the possibility that he might get to see all four Legitimate Successors in action at the same time. For that reason, he suggests to Yoshimori that they will need all the help they can get.

Suddenly, many butterflies surround Yoshimori's Shikigami bird and destroy it. Yoshimori is confused, since he felt no evil aura or transformation. With the increase in the number of butterflies, a few people such as Mr. Kurosu are suddenly able to hear parts of the message the butterflies are relaying. The mysterious girl who summoned the butterflies appears concerned. Tokiko rushes to the school, only to see tons of butterflies settling over it.

Characters (in order of appearance)

 * Yoshimori Sumimura
 * Sen Kagemiya
 * Tokine Yukimura
 * Shu Akitsu
 * Tokiko Yukimura
 * Kyoko
 * Yurina Kanda
 * Ayano
 * Madoka
 * Shigemori Sumimura
 * Mr. Kurosu
 * Mysterious Girl