Episode 10 of "When the Phone Rings" resumes with Hee-joo lying unconscious in the rear of Do-jae's vehicle, her mind wandering into a fantasy where she and Sa-eon share a blissful, ordinary life, untouched by the complexities that entangle their reality.
Meanwhile, the kidnapper arrives at the fish shop, anticipating an update from his employer, Do-jae. Instead, Do-jae confronts him with intentions far from benevolent – to confront the kidnapper head-on. Through a series of flashbacks, we piece together Do-jae's realization: the kidnapper is none other than the genuine Paik Sa-eon, a rogue partner who often defied their shared objectives. With a swift motion, Do-jae wields a bat, striking the kidnapper with brutal force.
Another flashback takes us to the day when Sang-hoon telephoned Sa-eon's office, with Young-woo intercepting the message. Despite Young-woo's attempts at secrecy, Do-jae's meticulous strategies led him straight to Juan Fishing. Upon arrival, he eavesdrops on an argument between the kidnapper and Sang-hoon, confirming his suspicion – the kidnapper is the real Sa-eon, the perpetrator who ended his twin brother's life.
In the present, a fierce battle unfolds between Do-jae and the kidnapper, but the latter's strength overpowers Do-jae. Brutally beaten with the metal bat, Do-jae is left defenseless and bundled into a black bag on the boat. Sa-eon arrives to find Do-jae seemingly enclosed in a black body bag, mistaking it for Hee-joo, and rushes to her supposed rescue on the boat.
The kidnapper attempts to flee with Hee-joo still unconscious in the backseat. However, she awakens and struggles against him, causing the car to swerve off the road and crash into a rock. Hee-joo escapes the wreckage and flees into Sa-eon's embrace. The kidnapper reverses the car, intent on ending both Hee-joo and Sa-eon's lives, but Sa-eon swiftly maneuvers them to safety.
Sa-eon confronts the kidnapper, who offers a deal: if Sa-eon desires his name and all that comes with it, he can take them, for he never wanted them in the first place. Boldly, the kidnapper demands Hee-joo as his wife, prompting Sa-eon into a rage, punching the kidnapper relentlessly. The kidnapper retaliates with a knife, but Do-jae, intervention saves Sa-eon, absorbing a fatal stab to his stomach in the process.
While the kidnapper is treating his wound in a bathroom, the scene cuts to when Kyu-jin finds him at Paik Jang-ho’s house. Kyu-jin knew that the kidnapper was her son. The kidnapper tells her that he had regained his memories a year ago after Jang-ho’s death. Seeing his grandfather’s face all over the news channels reminded him of the time he tried to drown him. Kyu-jin tells the kidnapper that Jang-ho had tricked her into believing that her real son was dead. Suddenly, Mr Min arrives with Ui-yong’s men and the kidnapper is forced to escape. Back in the bathroom, the kidnapper vows to make all of them pay.
Meanwhile, Kyu-jin is anxiously waiting for Ui-yong to find their son, but he has no intention to. While Kyu-jin thinks their son has suffered enough, Ui-yong believes he is making their lives a living hell. Ui-yong asks Kyu-jin if she can proudly announce the kidnapper as their son to the world or if she can take care of him all her life. Ui-yong thinks their son is nothing more than a monster they created.
Kyu-jin takes a jab at Ui-yong. She tells him that their son takes after him in many ways, but neither he nor their son takes after Jang-ho. However, Sa-eon takes after Jang-ho and he is rising faster than Ui-yong. Kyu-jin reminds Ui-yong that the Paik family values the presidential race more than their life and there is nothing else that Ui-yong can do if he has to give up running for President.
The police investigate Do-jae who is now recovering at the hospital. However, Do-jae refuses to cooperate. Sa-eon tries to get him to help catch the kidnapper, but Do-jae thinks Sa-eon cannot agree to expose the secrets of the Paik family and give up his privilege. When Do-jae asks if Sa-eon even saw through the facade and maybe started to suspect him, Sa-eon assures Do-jae that he trusted him. Even though he does not trust easily, Sa-eon completely trusted Do-jae. Sa-eon asks Do-jae to trust him too so they can catch the kidnapper.
Elsewhere, You-ri accompanies Sang-woo to the police station, where he identifies the belongings of the children abducted from the children’s home. At home, Hee-joo updates I-na about the switch between the Sa-eon and the kidnapper. I-na remembers that their little brother Yu-jin might have seen the face of the real Sa-eon when they were younger and it could be the reason why he was killed in the accident.
When Hee-joo’s mother comes to scold her about the recent events and how it might cost Ui-yong the presidency, Hee-joo fights back for the first time. She tells Yeon-hui she will never be the president’s daughter-in-law. Hee-joo says that she now understands love and knows that Yeon-hui has never loved her. Something Yeon-hui says about wishing she had married Hee-joo off to the son of Yeonhwa Construction helps I-na have an idea of who might have sent her the DNA report.
When Sa-eon arrives home, he finds the house lined with candles. Hee-joo emerges from the bedroom all dressed up and she hands their marriage contract. Hee-joo knows that Sa-eon was the one who sent the DNA report. Hee-joo was never the stand-in, but the one who Sa-eon wanted to marry all along. Sa-eon promises an official proposal to Hee-joo after they overcome their current difficulties and the couple embraces and kisses.
At the hospital, Do-jae receives a police report about the remains found at the gravesite. Do-jae cannot hold his tears back when he sees his brother’s shoe among the found items. Do-jae agrees to help Sa-eon catch the kidnapper. Luckily, Do-jae had installed trackers in everything he had given to the kidnapper so the detectives could use them to find his location.
Elsewhere, I-na and Hee-joo confront their father about the tragic accident that took the life of their infant brother. Upon discovering that the genuine Sa-eon is still alive, II-kyeong bursts into Jang-ho's memorial service, wielding a gun and menacing Ui-yong to surrender the real Sa-eon. Suddenly, an explosion rips through the air, orchestrated by the sinister kidnapper. Kyu-jin, who had spotted the kidnapper lurking outside the venue, unfortunately chose to remain silent. With a premonition of impending doom, Hee-joo swiftly sends a text to Sa-eon, urging him to come to the location.
As he arrives, the detectives simultaneously relay the kidnapper's whereabouts to him, which coincidentally is the same place. Sa-eon dashes inside, but Hee-joo is nowhere to be found. The kidnapper calls him, revealing that he has Hee-joo captive in his car, a knife hovering menacingly over her. The kidnapper vows to inflict pain upon Sa-eon for witnessing Jang-ho's drowning and failing to intervene.
Sa-eon races to the kidnapper's location, reassuring Hee-joo to stay strong and that he will rescue her. Yet, Hee-joo halts him, recalling her solemn vow to shield him. She professes her love for Sa-eon and vows to ensure he is no longer tormented by the kidnapper. With determination, Hee-joo floors the accelerator, steering the car intentionally off a cliff into the river. Sa-eon's ears are left with the final thud, followed by an eerie silence.
The police discover Hee-joo's car, but both she and the kidnapper remain elusive. Two days hence, Sa-eon holds a press conference, initially unveiling that he is not Ui-yong and Kyu-jin's biological son and that Hee-joo is his wife. He passionately pleads with the public to unite in bringing his beloved wife back home safely.
The episode concludes with a revisit to a romantic moment at Sa-eon's home. He confesses to Hee-joo that he was destined to become Paik Sa-eon solely to cross her path, and Hee-joo embraces him warmly.
