Netflix’s ‘Mousetrap’ Ending Explained: Who Is The Rat and What Happens to Moon-jae?
Based on the 2017 Kakao Webtoon Field Mouse by Ludovico, the South Korean thriller Mousetrap arrives on Netflix as a 10-episode cat-and-mouse game centered on the chilling concept of “identity laundering”. Directed by Kim Hong-sun (Project Wolf Hunting) and written by Lee Jae-gon (A Model Family), the series follows a reclusive novelist whose life is hijacked by a mysterious doppelgänger known as The Rat. Below, we break down the core mysteries, character connections, and that ambiguous final scene. Spoilers ahead.
What is Mousetrap about?
Reclusive bestselling author Je Moon-jae (Ryu Jun-yeol, Reply 1988) has lived in total isolation since his parents’ tragic death, his only link to the outside world being his accountant friend Son Su-hyeon (Moo Jin-sung). When Moon-jae suddenly loses access to his bank accounts and phone, he realizes his identity has been completely stolen by a figure who looks exactly like him. The impostor takes the moniker “The Rat” from a Korean folktale warning that a mouse eating a person’s fingernail clippings can transform into their likeness.
As Moon-jae becomes a fugitive, hardened detective Park Sun-yong (Lee Kyu-hyung) receives an anonymous tip about a body in the Han River, pointing toward loan shark No-ja (Sul Kyung-gu) — Moon-jae’s unlikely ally. No-ja is simultaneously pursued by identity fraud ring boss Park Seung-gyu (Kim Sung-oh), setting off a multi-layered chase.
Who is The Rat?
By the finale, we learn The Rat is Seo Yu-min (also played by Ryu Jun-yeol), a man Moon-jae met in an intercollegiate literary club. However, the thematic “Rat” turns out to be Moon-jae himself — an unreliable narrator who has repressed his past actions due to his late mother’s medication.
Flashbacks reveal Moon-jae infiltrated the literary club by posing as a university student, desperate to become a better version of myself, like my mom always told me
. He grew close to Yu-min and their friend So Min-yeong (Song Sui), both of whom were secretly a couple. Yu-min shared his unpublished story The Rat That Ate Fingernails
and his notebooks with Moon-jae, who stole the material and submitted it as his own work for the club anthology. The series underscores this theft with an early scene: Moon-jae stares at a blank screen, surrounded by notes, unable to type. None of it was ever mine to begin with
, he realizes in the finale.
Is Moon-jae actually Moon-jae? The detective’s hidden past
The identity puzzle deepens with Detective Park Sun-yong. Throughout the series, Sun-yong feels an eerie familiarity with the name “Je Moon-jae.” We learn he was adopted by Moon-jae’s parents at age five and given that very name. A flashback to a theme park shows Moon-jae’s uncle Han Hui-su calling the young Sun-yong “Moon-jae.”
When the couple later had a biological son, they sent Sun-yong back to the orphanage and gave their new child the same name — Moon-jae. This means Sun-yong has been unknowingly hunting his own adoptive brother. Yu-min knew this, which is why he personally tipped off the detective. In the finale, Moon-jae recalls starting a fire at a school under the alias Park Chan-u, explaining: It doesn’t feel like my name
, despite not knowing it was originally his brother’s.
What happened to Min-yeong? The tragedy that sparked the revenge
The series’ emotional core lies in the fate of Min-yeong. During a summer trip to Jeju Island, Moon-jae — drunk and jealous of Yu-min and Min-yeong’s closeness — attacked Yu-min after she rejected his confession. Later, in an envious rage, Moon-jae fabricated explicit details of their relationship in an online forum post. The resulting public shaming drove Min-yeong to drop out of graduate school and cut ties with her family. Yu-min desperately tried to prove Moon-jae authored the post, but You must have been one hell of a writer. Even the cops had nothing on you
, Yu-min tells him in the finale.
Moon-jae dismisses it: I was high on drugs. Who’d read that bullshit? I just wrote it like a novel
.
Yu-min supported Min-yeong emotionally and financially, sending his novel The Rat That Ate Fingernails to a publisher. Instead, he discovered his work had already been published as Interview by Sung Hag-Yun — winning Moon-jae a prestigious Best New Writer award and launching his career. Devastated, Min-yeong died by suicide. Yu-min read Interview beside her body and recognized their lives twisted into Moon-jae’s perspective.
You were so jealous of me that, in your novel, you subtly made yourself me and me you, Yu-min says.So I decided. I’ll get my revenge. I’ll become you too and take back everything you took from me. I’ll rip that mask off your face, strip you of everything, and throw you out.
What’s the Rat’s plan? A calculated identity theft
After Min-yeong’s death, Yu-min undergoes plastic surgery to replicate Moon-jae’s face and trains to mimic his voice. He infiltrates Seung-gyu’s identity fraud ring, selling his own identity but faking his death before it could be laundered. He also sold Min-yeong’s identity to Seung-gyu, murdered a random woman, replicated Min-yeong’s tattoo on the corpse, and tipped off police to frame Moon-jae for murder while implicating Seung-gyu.
Yu-min forced Sun-yong to assist by kidnapping the detective’s daughter, compelling him to clone Moon-jae’s phone and block his finances. Su-hyeon dies by suicide before Moon-jae can extract answers. University records ultimately reveal the truth: no record of Min-yeong (her identity was sold), no record of Moon-jae (he wasn’t a student), but a record exists for Yu-min.
Are both Moon-jae and Yu-min alive at the end?
The finale converges on the Queen Caligo, the ship bound for Jeju where the trio’s fateful summer unfolded. In the same cabin, Moon-jae confronts Yu-min, learns the full truth, and throws him from the window into the sea. In that moment, Moon-jae finds his motivation: I don’t think I wanted to be you. I think I wanted to be myself. A better version of myself.
The true power broker is publishing house editor-in-chief Han Ji-hye (Lee El, The Call), the only person who spoke to Moon-jae in three years. Her observation allows her to distinguish the two. After Yu-min confesses, she hands Moon-jae the unpublished manuscript of The Rat Vol. 2, which exposes Moon-jae’s crimes — essentially the story of Mousetrap itself. This should never get out
, a panicked Moon-jae says in voiceover.
In the final scene, loan shark boss Seung-gyu pounds on Moon-jae’s apartment door. In the square below, Yu-min appears — alive, or perhaps a guilt-induced hallucination. Either way, Moon-jae’s attempt to outrun his past has failed.
Mousetrap is now streaming on Netflix.
