The end of World War II. Facing certain defeat, the Imperial Japanese Army realized they could no longer turn the tide of war with conventional weaponry.

Amidst this, a single plan was proceeding in absolute secrecy. No official records remain, and those involved vanished in the post-war chaos— A "plan that was never supposed to exist."

Its name:

The "Diamond of Kaiten" Project.

It wasn't even a weapon. Rather, it was something that could be made into a weapon.

An iridescent, shimmering crystalline body. Changing colors depending on the angle of light, the stone looked as if it held a tiny universe trapped inside, harboring energy that far exceeded common sense.

Seven thousand times— That of modern nuclear power.

Furthermore, it produced no radioactive waste and showed no signs of going out of control; it was a theoretically "perfectly clean" energy source.

If this had been put into practical use, not only the outcome of the war, but the very history of humanity might have been rewritten.

But— The project was cut short just before completion.

Defeat in war. Incinerated documents. Silenced researchers.

The "Diamond of Kaiten" was buried in the darkness of history along with its very existence.

...Or so it was thought.

---

Time passes, to the present day.

Guest was just an ordinary person living an ordinary life. No special powers, no special mission.

Except for one thing.

A pendant given to them by their parents when they were young. It was a simple piece of jewelry that looked like something you could find anywhere.

However, the way its brilliance shimmered slightly depending on the light gave off a sense of wrongness, as if it were somewhat detached from reality.

And one day—

Its true nature is revealed.

The small fragment used in the pendant. That was a piece of the secret stone that was supposed to have vanished from the world—

A part of the "Diamond of Kaiten."

---

After the war, the scant information regarding the secret stone quickly spread through the "underworld."

Intelligence agencies of various nations began to move.

National-level surveillance networks.

Infiltration agents.

Plans to deploy special forces.

The struggle for the "Diamond of Kaiten" had already begun.

It is not a mere resource. It is a "key" with the potential to solve energy problems, collapse the military balance, and even rewrite global hegemony.

Naturally, it isn't just nations that are after it.

---

Rumors reach the world of shadows as well.

The world-class master thief, Lupin III and his gang.

"A gem harboring seven thousand times the energy." —To them, it was a "prey" far too alluring to ignore.

---

And so, all eyes gather on one person.

Guest, who was supposed to know nothing. But on their chest lies the "fragment" that could shake the world.

In a conflict with no escape, it will eventually become clear.

That this stone— Is not just a simple energy source.