The ending of Mousetrap finally answers the big question that has been hanging over Netflix’s Korean thriller: who exactly is “The Rat”? For most of the ten episodes, it seems like the mystery is mainly about Je Moon-jae, the writer played by Ryu Jun-yeol, trying to figure out who stole his name, his money and basically his entire life. The answer is Seo Yu-min, someone Moon-jae knew from his university literary club. But that reveal is only part of what the finale is doing.
The real twist is the reason Yu-min stole Moon-jae’s identity in the first place. As Moon-jae’s memories start coming back, we learn that he wasn’t quite the innocent victim the story initially made him out to be. His own career was built on stolen ideas, stolen words and, eventually, someone else’s life. Directed by Kim Hong-sun and written by Lee Jae-gon, and based on Ludovico’s webtoon Field Mouse, Mousetrap turns identity theft into something much bigger. By the end, the question isn’t simply who “The Rat” is, but who actually deserves that name.
Who Is “The Rat” in Mousetrap?
The person known as The Rat is Seo Yu-min, also played by Ryu Jun-yeol. Moon-jae met him years earlier through an intercollegiate writing group, where the two became friends along with So Min-yeong. Moon-jae had joined the club while pretending to be a university student, partly because of the pressure from his mother to become “a better version of himself.” He eventually developed feelings for Min-yeong, not realizing that she and Yu-min were already secretly together.
That relationship becomes important because Moon-jae eventually gets access to Yu-min’s private notebooks. Inside them is a draft called “The Rat That Ate Fingernails.” Moon-jae takes the work and submits it as his own for the literary club’s anthology. In other words, the first identity theft in the story happens much earlier than we realize. Moon-jae is already taking something that doesn’t belong to him before anyone steals anything from him.
Things get much uglier during a trip to Jeju. Moon-jae discovers Yu-min and Min-yeong’s relationship and, consumed by jealousy, attacks Yu-min. He then creates a fake online post containing explicit details about the couple. Because Moon-jae is so good at turning reality into something that sounds like fiction, people believe the story. Min-yeong is humiliated, becomes isolated from everyone around her, drops out of school and eventually cuts ties with her family.
Yu-min tries to help Min-yeong financially by submitting his unpublished manuscript, “The Rat That Ate Fingernails,” to a publisher. Instead, he discovers that Moon-jae has already stolen the story and turned it into Interview, the novel that makes his career. It becomes a huge hit and wins Moon-jae a major literary award. Then comes the worst part: Min-yeong dies by suicide. Yu-min finds her with Moon-jae’s novel, and reading it beside her, he realizes just how much of his and Min-yeong’s lives have been taken and turned into Moon-jae’s success.
At that point, Yu-min decides that if Moon-jae stole his life through his writing, he will return the favor in the most literal way possible. He takes Moon-jae’s identity, wealth and status, essentially stripping him of the life he built from Yu-min’s talent in the first place. It isn’t just revenge for a stolen novel anymore. Yu-min wants Moon-jae to experience what it feels like to watch somebody else live the life that was supposed to be yours.
So Who Is the Real “Rat”?
This is where the finale gets considerably more interesting. Yu-min may be the person physically carrying out the identity theft, but Moon-jae was actually “The Rat” first.
Several details from earlier episodes suddenly make more sense because of this. One of the most telling is Moon-jae repeatedly sitting in front of his computer, surrounded by notes and ideas, yet completely unable to write. His writer’s block isn’t simply a creative crisis. He has spent his career pretending to be a writer while having very little of his own to say. The success everyone associates with him was built on Yu-min’s work.
The title itself also comes from an old piece of Korean folklore in which a rat eats a person’s fingernails and takes on that person’s physical appearance. Mousetrap basically takes that idea and gives it a modern twist. Nobody needs magic to become someone else anymore. Steal their words, their story, their reputation and their public identity, and you can get pretty close. Yu-min’s revenge works because he isn’t simply destroying Moon-jae; he is making him live through the same nightmare he once inflicted on somebody else.
Then the finale adds one more identity twist with detective Park Sun-yong. Sun-yong reveals that he was adopted by Moon-jae’s parents when he was young and was originally given the name Je Moon-jae. But when the couple eventually had a biological son, they sent Sun-yong back to the orphanage and gave the name to their new child instead. So the detective hunting Moon-jae has unknowingly spent his life chasing a man who took his name from him before he was even old enough to understand what had happened.
And that is really the point Mousetrap is making with all of these identity swaps. Names don’t necessarily tell you who a person is. Sun-yong was originally Je Moon-jae. Moon-jae once went by Park Chan-u. Yu-min eventually became Moon-jae. And the man at the center of everything built his entire literary career by taking someone else’s work and, in a much broader sense, someone else’s life.
So by the end, “The Rat” isn’t really just one person. Yu-min is The Rat, technically, but Moon-jae was one long before him. The title ultimately becomes a description of anyone who survives by feeding off another person’s life and then convincing everyone that what they’ve taken was theirs all along.