Mousetrap Episode 1 begins with a dark memory from Je Mun-jae's childhood. His parents, Ju Geun-u and Han Yeong-hui, died in a tragic incident after his mother drove their car into the sea, with his father's body never being recovered. Before her death, his mother had left clipped fingernails behind, a disturbing detail that becomes deeply connected to the Korean folktale at the heart of Mun-jae's life. The trauma leaves Mun-jae with severe anxiety and panic attacks, eventually pushing him toward a completely isolated existence. Years later, Mun-jae is living alone in a luxurious apartment and has barely left his home for three years. He is actually a highly successful novelist, but he publishes under a pen name and keeps his real identity hidden from the public. His life is built around strict routines designed to keep the outside world away. He suffers recurring nightmares and sudden panic attacks, while his only trusted friend, Son Soo-hyun, handles many of the practical matters of his life. Then everything begins to unravel. Mun-jae discovers that his fingerprint no longer unlocks his phone. His passwords fail, and the systems that once recognised him suddenly treat him like a stranger. When he returns home, he finds that his belongings have disappeared. A security guard tells him that someone came earlier, repaired the damaged window and moved everything out. Mun-jae is stunned because there appears to be no way for him to prove that the life he claims is actually his. At the same time, Detective Park Sun-yong is investigating the mysterious death of an unidentified woman whose body has been recovered from the Han River. The medical examiner finds no obvious signs of foul play, but the investigation gradually connects to a dangerous gangster known as No-ja. No-ja is a ruthless loan shark and the head of an organisation involved in illegal gambling, while also operating a private investigation agency. He has his own reason for searching for Mun-jae: Mun-jae borrowed money from him three years earlier and then disappeared. Desperate for answers, Mun-jae starts looking for information about his missing friend Soo-hyun. His search brings him face-to-face with No-ja. The encounter quickly turns violent as No-ja attacks him, forcing Mun-jae into a frantic escape. When police arrive, No-ja retreats, while Mun-jae hides nearby and is overwhelmed by another severe panic attack. His physical fear and inability to trust his surroundings make the situation even more terrifying. The episode also returns to Mun-jae's past, showing how his fear and isolation developed after moving into his apartment. His carefully controlled existence has protected him from the outside world, but it has also left him dangerously dependent on a small number of people he trusts. Now those connections are disappearing one by one. The mystery reaches its biggest turning point when Mun-jae encounters evidence that someone has not merely stolen his money or possessions but has taken his entire identity. The person appears to know his life, his relationships and the details needed to pass as him. Even more horrifyingly, Mun-jae eventually comes face-to-face with a man who looks like him. His own appearance is suddenly no longer proof that he is Je Mun-jae. Episode 1 establishes the central nightmare of Mousetrap: a man who has spent years hiding from society suddenly discovers that society recognises another person as him. With his name, property and identity slipping away, Mun-jae must find a way to prove that he is the real person before the mysterious figure known as 'The Rat' completely replaces him. The episode ends by turning the identity mystery into a dangerous pursuit, setting Mun-jae on a collision course with No-ja and the mysterious double who appears to have stolen his entire life.