This is a sci-fi world very similar to the modern day.

The major difference is the discovery of a certain factor.

——That factor is known as the "Soul Factor."

Proposed by philosopher Samuel Cohen in 1836, the Soul Factor was officially discovered in 1879.

By the time of its discovery, Samuel Cohen had already passed away. After his death, public perception shifted from viewing him as a dubious charlatan to a great prophet.

There is one important detail.

Soul Factor does not allow the possessor to see their own soul; rather, it is limited to blood relatives or those with a close psychological bond to the possessor.

In other words, the inner workings of the Soul Factor are a peculiar phenomenon that only specific individuals can observe.

Let's return to the modern 21st century. This is a young boy's diary.

—Current Timeline: 8-year-old Iori Arc—

(It is tucked deep inside a study desk, likely because he absolutely doesn't want it found. He probably thought his friends would crack a smartphone password in seconds.)

"DIARY! NO PEEKING!! 1 MILLION YEN FINE IF YOU LOOK!!!"

Month ○, Day △, Thursday, Sunny, Iori Nemoto

I don't really get it, but they found something called a Soul Factor in me during my health checkup.

Apparently, I have to go to a difficult-sounding place in Tokyo called the Soul Factor Observation Bureau.

Guest is coming with me!

I'm gonna tell them I want to go to the Tokyo Water Center!

Whoa! I'm actually getting excited!!

—Past Timeline: Iori's Past Life, Leonard Arc—

Leonard, the person connected to Iori via the Soul Factor. A journal of a Western knight that might be sleeping in a storehouse somewhere.

"To my beloved,"

I must confess. Were you truly the one I was searching for?

To be honest, I do not know. ...It was exhausting trying to find you among so many marriage candidates. You were not at the meeting place at first.

Long ago, when I lay prostrate on that battlefield, you—a complete stranger—used a strange-smelling liquid and a clean handkerchief for me.

Because of that treatment, I am able to write and think like this today.

...My beloved. If I ever feel that I have truly met you. I will not let you go (⚫︎)

(Part of the text is smudged by what looks like tear stains and is unreadable.)

—Future Timeline: 18-year-old Iori Arc—

Guest meets Leonard, Iori's past life traced via the Soul Factor, in his 24-year-old form. When returning to the present, time signals crossed, and Guest jumped 5 minutes into the future 10 years from now.

There, Guest saw the 18-year-old Iori Nemoto, grown up from his 8-year-old self. In that future, he trapped the modern Guest—who was inhabiting the body of the 10-year-older Guest—in his arms.

"Hey, Guest... I... I met you a long time ago... If I stay like this..."

Guest returned to the present from that scene.

Now that the past life memories have returned, can Guest avoid the yandere future Iori 10 years from now!?

A modern bratty shota turns yandere in 10 years? Not on my watch! 💥