It’s a Japanese manga series written by Miura Tsuina-Sensei and illustrated by Ooba Takahiro-Sensei, serialized online in DeNA’s Manga Box app from 2013.

It’s concluded in 2019 and as for comics, 21 volumes have been released. Currently, a new series, “Tenkū Shipan Arrive” has been serialized, spreading its worldview even more.

The main protagonist, Honjo Yuri, trembles in fear and confusion.

She is supposed to be at school. But now, she finds out she’s trapped in a strange and abandoned building. And……

She is now in the site, where a masked assailant is cruely murdering the office worker.

As soon as she realizes that she’s there, she starts to escape.

She keeps running, thinking that she really needs a help from someone, letting them know that the man is murdering a person…… when she finds a strange sign, saying “You cannot go down to the ground.”

Looking behind, she sees the masked assailant, slowly chasing her.

Yuri desperately runs away from him, ending up getting to the rooftop. Then Yuri feels strange, looking at the view from there.

It’s the rooftop, surrounded by high-rise buildings.

Looking down from there, Yuri realizes that she sees neither vehicles nor people on the street and…… There’s no way she can stay alive if she “falls down” from there.

Yuri is sure that it’s the “emergency” and calls the police at once. However, she cannot get hold of anyone, neither her parents nor the police. Though filled with despair, she is able to talk with her older brother on the phone.

He says such terrifying things to her:

First of all, he is also in this “same” strange place, surrounded by high-rise buildings, even though it’s not the exactly the same location. And this might be a “reality” and they can never go down to the ground through the stair of the building…… but they can move between the buildings because all the buildings are connected by the suspension bridges.

Yuri suggests that they should meet in the tower, which stands out among all the high buildings. They both finally get to the tower and are about to see each other……

…… when a masked man is behind.

She is able to run away from him but she accidentally drops her cell phone through a gap between the buildings when she dodges his attack.

High-rise buildings, murderers, the different world……

Even though my brother says it’s a reality…… I might be able to “wake up” from this world if I fall down to the gap between the buildings, by just loosening my grasp, holdign the suspension bridge……?

Yuri scolds herself, who has thought about that idea even for a moment.
My brother is waiting for me. And I “do not want to die”!!!!!!

This is how she determines to find him and escape, risking her life.

Yuri is now moving from the building to building and seeing various people and the masked murderes. What on earth is this world? Why are all the murderers masked……!? Is Yuri able to survive and find out these mysteries at the end……?