Mousetrap Episode 4 begins with Moon-jae and Noja desperately trying to catch up with Son Soo-hyun. When they finally locate him, Soo-hyun reacts with panic and attempts to escape. Moon-jae chases after him, demanding answers, while Noja tries to stop the situation from getting out of control. Soo-hyun eventually reveals a terrifying truth: the mysterious man calling himself The Rat told him that he knew Moon-jae extremely well and had been preparing to take over Moon-jae's life for a long time. Soo-hyun explains that he had become involved in a financial fraud involving stolen investment funds. The Rat appeared and knew about the money, the people involved and the details of Soo-hyun's personal life. He threatened to expose Soo-hyun to the police and used his daughter Da-eun as leverage. When Soo-hyun refused to cooperate, The Rat kidnapped Da-eun and threatened to kill her if Soo-hyun was caught or revealed what he knew. The revelation shocks Moon-jae because Soo-hyun confirms that The Rat knew intimate details about their relationship. He also reveals that The Rat deliberately changed his voice so that Moon-jae would not recognise him. The entire identity theft was not improvised. The phone, the broken window and the events surrounding Moon-jae's disappearance were all part of a carefully designed operation. The Rat had been preparing to take everything Moon-jae owned and replace him with another person. Soo-hyun also reveals that he was ordered to bring the phone he had been using. The phone contains information that The Rat needs, making it another important part of the larger scheme. Moon-jae finally understands that the strange events surrounding his apartment and missing belongings were connected from the very beginning. Someone had planned the entire sequence in advance and had been manipulating the people around him to make the identity theft possible. Noja confronts Soo-hyun about the money that was stolen from Dong-seok. Their conversation exposes another layer of the criminal network. Noja learns that an anonymous tip had brought unwanted police attention to them, putting the entire organisation at risk. Soo-hyun warns Noja that he is also in danger and tells him to stay out of sight. Noja dismisses the warning, but he soon realises that the police really are watching him. Detective Park Sun-yong's investigation continues to tighten around Noja and the people connected to the identity operation. The police surveillance becomes a serious problem because Noja cannot move freely without risking the safety of himself and Moon-jae. After discovering that he has been followed, Noja takes action to shake the tail and then tells Moon-jae that they need to leave immediately. As they prepare to move, Moon-jae remains desperate to understand who The Rat really is. The clues force him to look beyond the recent identity theft and into his own past. A recovered memory makes him suspect that The Rat may be someone who knew him long before his current life disappeared. The possibility is especially disturbing because it means the person behind the crime may have understood Moon-jae's history, fears and relationships for years. Moon-jae and Noja follow this new lead to an island connected to the man's past. Their journey changes the investigation from a search for a random identity thief into a search for someone personally connected to Moon-jae. The island becomes a crucial location because it contains memories and clues that may explain why The Rat knows so much about him and how the replacement identity was created. But before Moon-jae can get all the answers, The Rat makes direct contact. A mysterious man arrives and gives Noja a phone, instructing him to use a specific speed-dial number. When the call connects, The Rat speaks directly to Moon-jae. Moon-jae demands to know who he is, but The Rat responds with chilling confidence, telling him that he should already know the answer. Then comes the episode's most disturbing revelation. The Rat tells Moon-jae that he is him. He challenges Moon-jae to figure out who he really is, suggesting that understanding his own identity is the only way he can understand The Rat's identity. The call turns the entire mystery on its head: Moon-jae is no longer simply searching for an impostor who stole his life. He is being forced to question whether he truly understands his own past and the person he has always believed himself to be. Episode 4 therefore pushes the mystery much deeper. Soo-hyun's confession proves that The Rat's operation was carefully planned, Noja discovers that the police are closing in, and Moon-jae's recovered memories point toward someone from his past. The episode ends with The Rat directly entering the psychological game and claiming that he is Moon-jae himself, leaving Moon-jae with the most frightening question yet: if The Rat knows everything about him, how can he prove which one of them is real?