- Seo Yu Min becomes The Rat and steals Moon Jae’s identity as revenge for the damage Moon jae caused.
- Moon Jae is also The Rat because he stole Yu Min’s writing and manipulated the truth about their past.
- Detective Sun Yong was originally adopted as Je Moon Jae, making the identity mystery even more complicated.
- Yu Min’s fate is deliberately left uncertain, but Moon jae clearly cannot escape his guilt or his past.
Netflix’s South Korean thriller Mousetrap saves its biggest twist for the very end. What initially looks like a straightforward story about a novelist whose identity has been stolen gradually turns into something much darker. By the finale, the question is no longer simply who The Rat is. It becomes a question of memory, guilt, stolen identities and the lengths people will go to rewrite their own past.
The story follows novelist Je Moon jae, a wealthy but isolated writer whose life suddenly begins falling apart. His bank accounts stop working, his phone becomes inaccessible and the police start treating him as a suspect in a murder case. Worse still, someone who looks exactly like him appears to be living his life.
That person is eventually revealed to be Seo Yu min. But even that answer is only part of the truth.
Who is The Rat in Mousetrap?
Yu min is the person behind the identity theft scheme. He has undergone plastic surgery to resemble Moon jae and has carefully copied his voice, mannerisms and personal details. His goal is not simply to steal Moon jae’s money. He wants to destroy the identity and reputation of the man who ruined his life.
The connection between the two men goes back to their university days.
Moon jae had never really been the successful writer he later presented himself as. He joined a literary club while pretending to be a university student because he wanted to become the person he believed his mother expected him to be.
There he met Yu min and So Min yeong. Yu min and Min yeong were already secretly involved, but Moon jae developed feelings for her.
Yu min also made the mistake of trusting Moon jae with his writing. He showed him his story, The Rat That Ate Fingernails, along with his notebooks. Moon jae eventually stole the material and used it as the foundation for his own literary career.
That detail explains one of the most important clues in the series. When Moon jae later sits in front of a blank computer screen, unable to write, it reflects the truth he has spent years avoiding. Much of what made him famous was never really his.
So, yes, Yu min is The Rat in the literal sense. He is the man who takes Moon jae’s identity and attempts to replace him.
But the show gives the title a second meaning.
Moon jae is also The Rat.
Moon jae is an unreliable narrator
The biggest revelation is that Moon jae has forgotten or distorted parts of his own past.
His mother’s medication plays a major role in this. His memory is unreliable, which allows him to construct a version of his life in which he is more victim than aggressor.
The truth is considerably uglier.
Moon jae was jealous of Yu min and Min yeong. During their trip to Jeju, he became drunk and confronted Min yeong about his feelings. She rejected him and revealed that she and Yu min had been together before he ever met them.
Moon jae then attacked Yu min.
His behaviour became even more destructive later. He wrote an online post containing fabricated sexual details about Yu min and Min yeong. The writing was convincing enough to make the accusations appear genuine. Min yeong was publicly humiliated and eventually abandoned her studies and family connections.
Yu min tried to prove that Moon jae was responsible, but there was not enough evidence to expose him.
The irony is brutal. Moon jae was a writer who became successful by turning other people’s experiences into his own stories, while Yu min eventually decided to turn Moon jae’s entire life into a revenge story.
Min yeong’s death becomes the emotional turning point. Yu min sends his unpublished novel to a publisher hoping it can give them both a way forward. Instead, he discovers that the story has effectively already been stolen.
Moon jae has published a book called Interview, which draws heavily from Yu min and Min yeong’s lives while changing their roles. Moon jae has essentially rewritten reality so that he becomes the central figure.
After Min yeong dies by suicide, Yu min decides that taking Moon jae’s identity is the only way to expose what really happened.
The family twist changes everything
There is another major identity revelation involving Detective Park Sun yong.
Sun yong was adopted by Moon jae’s parents when he was five years old. He was originally given the name Je Moon jae. When the family later had a biological son, they gave the new child the same name.
That means the detective has unknowingly spent much of the series investigating his own adopted family and pursuing the man who inherited the identity that once belonged to him.
It also explains why certain memories and names feel strangely familiar to him.
Moon jae himself has fragments of memories involving different identities. At one point, he remembers registering under another name and even starting a fire at school. He could not understand why the name did not feel like his.
The show uses these identity swaps to reinforce its central idea. Names, documents and appearances can be changed, but the past remains underneath them.
What happens to Yu min at the end?
The final confrontation takes place aboard the Queen Caligo, the ship travelling toward Jeju. It is an appropriate location because Jeju is where the chain of events between Moon jae, Yu min and Min yeong originally began.
Moon jae finally confronts Yu min and learns the full extent of what happened.
Yu min explains that his revenge was deliberate. He wanted to become Moon jae, take everything from him and expose the person hiding behind the successful author’s public image.
Moon jae eventually understands something important about himself. He says that he did not really want to become Yu min. He wanted to become himself, or at least a better version of himself.
The fight ends with Moon jae throwing Yu min from the ship into the sea.
That appears to settle the conflict, but Mousetrap refuses to give viewers a completely clean ending.
Han Ji hye, the publishing editor who has remained one of Moon jae’s few connections to the outside world, understands what has happened. Yu min has left behind another manuscript that tells the real story and exposes Moon jae’s crimes.
Moon jae is terrified that the manuscript will become public.
Then comes the final scene.
When Park Seung gyu reaches Moon jae’s apartment and starts banging on the door, Yu min appears outside. The show never confirms whether he survived the fall or whether Moon jae is imagining him.
That ambiguity is deliberate.
If Yu min survived, Moon jae’s nightmare is not over. If Yu min is dead, Moon jae is still being haunted by everything he tried to bury.
Either way, the ending reaches the same conclusion. Moon jae cannot escape his past simply by changing his name, hiding his memories or controlling the story.
The real trap was never just an identity theft scheme.
It was the story Moon jae told himself about who he was.
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