A breathtaking showdown between a police officer who has sworn revenge and Detective Gaga who is trying to stop him.
Pursuing the pinnacle of pure reasoning
Keigo Higashino's ultimate authentic mystery

"One of the Two Killed Her" is the fourth installment of the [Detective Kaga Series], and Kaga, who worked as a Metropolitan Police Department detective in the previous installment, "Malice," appears for the first time as a member of the Tokyo Nerima Police Station.
When publishing “One of the Two Killed Her,” Higashino declared his belief that “a true mystery novel is one where the reader does the guessing,” and he ended the novel intentionally without revealing the name of the culprit.
Although Higashino is recognized today as a master of social mystery, this work is filled with the mystery novelist's unwavering love for authentic mystery.

The culprit remains undisclosed until the very end. Can you figure out who he is?
At the time of publication, it was a controversial work that paralyzed the publisher with phone calls asking who the culprit was.

A woman found dead in her studio apartment.
The older brother, who first discovered the body, realizes that his younger brother has been murdered and vows to take revenge himself, concealing the evidence.
Through his own investigation, he narrows down the suspects to two.
It was her sister's ex-lover and her old friend who killed her!

Looking at the plot so far, one might wonder if it isn't a bit simple for Higashino's work.
However, this is only in accordance with the author's strict calculation to boldly omit any parts that would interfere with the reasoning.
Higashino has created a work that draws a clear line between itself and the average detective game, with a simple yet ingenious structure.

The tug-of-war between the police officer who is determined to take revenge, Detective Gaga who is trying to stop him, and the two suspects creates a tense atmosphere from start to finish.
And then, at the crucial moment when it is finally revealed which of the two killed her, the story ends.
Higashino deliberately leaves the name of the culprit undisclosed, leaving the reader to discover crucial clues for themselves (the appendix, in critic Shinta Nishigami's "Seal Commentary," brings these clues to mind again).

This work, which began with the question of whether readers can truly "infer", was possible because Keigo Higashino naturally draws out the reader's imagination by leaving a blank space at the center of the story.
When it was first published, it shocked so many people that the publisher was flooded with calls asking who the culprit was, and the publisher even had to create an 'answer manual'.
Even now, you can often find readers asking for the culprit, and this work, along with "I Killed Him," is the ultimate full-fledged mystery that requires readers to step forward and uncover the truth of the case.

'I think it would probably be best if I died.'
My brother must have been shocked to hear that.
Sonoko thought she had said something she shouldn't have.
But for her, it was an honest feeling.
Who among Junichi or Kayoko?
Sonoko had an ominous thought.
I wish one of them would kill me.
That was when.
The doorbell rang.
--- p.39~40

The moment he realized it was a murder, Yasumasa decided to uncover the culprit with his own hands.
There are things in the world that I should do with my own hands and things that I shouldn't.
He felt that this was not something he could leave in someone else's hands.
For him, his sister's happiness was the greatest wish in life.
The resentment of having it taken away can never be alleviated by the arrest of the culprit.
--- p.110

I thought his words, “I believe in you,” were not just a formality.
As he said, if he really wanted to stop Yasumasa's revenge, there would be plenty of ways to do it even at this point.
The reason he doesn't do it is because he clearly trusts Yasumasa's reason.
But, Yasumasa thought.
The detective is still young.
He does not know the existence of humans.
Humans are a bit uglier, more cowardly, and weaker.
Yasumasa tried to banish Kaga's voice, which had been speaking passionately, from his head.
I tried to focus solely on my work without thinking about anything else.
--- p.292~293

“I can’t say we have an answer yet,” the detective said.
“I guess so.
You have no choice but to say so.
But the answer came out.
“I witnessed that moment with my own eyes.”
Kaga took a deep breath at Yasumasa's words.
Yasumasa nodded at the sight.
"I hope you've found your answer now. It'll save you the trouble of having to refer the case to the forensic department."
And Yasumasa looked at the switch in his hand.
There was no more hesitation.
The truth has been fully revealed.
“What do you mean?” Kayoko’s voice rose.
“You have to explain it clearly!” Jun-ichi shouted.
His eyes were bloodshot.
Yasumasa chuckled.
“You don’t have to say anything anymore.
“The answer is out.”
“How did you get out of here!”
“You’ll know when you see it.” Yasumasa slowly raised the switches in his hands to face level.
“Now, which of the two will survive?”

--- p.376