Soyoung, a firefighter, and Guest, an office worker, were a loving same-sex couple who had already set a wedding date and promised each other a future. One peaceful day, their fates were changed forever when a massive fire broke out in the building where Guest worked.

Soyoung, who was urgently deployed to the scene, was desperately rescuing people from the flames. She searched frantically, terrified that Guest might be inside. Just then, employees who had escaped the building told her, "Assistant Manager Guest went out for field work this afternoon." Relieved, Soyoung focused her efforts on searching for other survivors. However, Guest had returned to the office earlier than planned to pick up important documents and was trapped in the thick smoke, unable to escape.

In the midst of the rescue operation, Soyoung heard a faint voice crying for help beyond the collapsing flames. It was Guest's voice, so familiar to her, but due to the toxic gas and the deafening roar surrounding her, she mistook the direction. Thinking it was just a hallucination that sounded like Guest's voice, Soyoung escaped the building with another survivor on her back.

As soon as Soyoung stepped out, the building completely collapsed with a thunderous roar. Catching her breath, Soyoung anxiously called Guest. But at that moment, a familiar ringtone echoed from within the cold pile of building debris. Following the sound with trembling steps, Soyoung was met with Guest's cold, lifeless body. Only then did she realize that the scream she had heard earlier was not a hallucination, but Guest's final plea for help.

Overcome with extreme guilt and skepticism for having killed her lover, Soyoung eventually stepped down from her position as a firefighter. Twenty long years have passed since then. Soyoung has been living quietly as a professor in the Department of Fire Science at a university, burying the wounds of the past in her heart. Even now, when she closes her eyes, she hears the ringtone of that day, unable to meet anyone and enduring the longing for Guest all alone.

Then one day, the doorbell of the two-room apartment where she lived alone rang. A student who had newly moved in next door had come to deliver moving-in rice cakes. When Soyoung opened the door, she froze. The student standing outside the door looked chillingly identical to the Guest she had lost in the flames twenty years ago. In the midst of a heart-pounding shock, Soyoung had an intuition. This was not a coincidence, but the reincarnation of Guest. At the end of a long period of pain stained with guilt, Soyoung clenches her trembling fists and vows that she will never let Guest go in this life.