In the days that followed the closing of the portal, when Tae-eul was still reeling from the loss of Shin-jae and the knowledge that she might never see Lee Gon again, she was woken each night by the sound of a flute. The music was soft but persistent, as though its player was standing just out of sight. It didn't stop when Tae-eul got out of bed and turned on the light, or when she asked if anyone was there. It simply continued playing, as though she were still dreaming and the rules of the real world were temporarily on hold.

She'd tried to follow the sound at first. It had led her out the door and around the block, but disappeared when her slippered feet circled back past the drive.

"I'm going mad," Tae-eul thought miserably to herself. She supposed it wasn't surprising considering her recent involvement with time travel. Still, it hurt to know that on top of what she'd already lost, she was now losing a piece of herself. "Can you explain this to me, Gon?" She asked into the darkness. "Did you know this would happen?"

She would have loved to hear his teasing voice at that moment. "Of course there could be side effects to standing inside a broken portal at the precise moment history is rewritten," she imagined him saying. "If the flute had never been cut in two, that unstable path would've never existed."

She wondered if her continued existence, or at least her memory of events, was a glitch in the grand scheme of the universe. "It would've been better to forget than go insane," she muttered to herself, then regretted it.

Even now, as the sole person on the planet who remembered Lee Gon and the loneliness that knowledge brought, she wouldn't have traded her memories for peace and quiet. "All those radiant moments, Gon... I won't forget." She whispered. "I can't."

As the weeks passed, Tae-eul found herself growing used to the strange melody in the background of her life. Sometimes she'd catch herself gazing out the window, or craning her neck to see past the crowd on a busy street, and she'd realise she was looking for a sign of him, and that the music had started again. Was it calling her? Or was she calling it? And where did it come from? She couldn't recall ever hearing a piece like that before. Was her brain really making it up?

She'd tried to test her sanity around others. "Can you hear that, seonbae?" She'd asked a senior at work. "What?" he replied through a mouthful of noodles. "The air conditioning? It's getting louder."

"No," Tae-eul shook her head, "the music."

The officer cocked his head to one side and listened. "Nope," he said after a pause, "But maybe your hearing is better than mine. I went to a lot of loud concerts when I was young."

She'd tried recording the music later, but her phone didn't register a sound.

It was getting harder and harder to concentrate at work. In the past she'd been excited to go on stakeouts and chase down suspects, but now her heart wasn't in it. It wasn't just loneliness that distracted her, but the knowledge that a whole dimension existed on the other side of a door, and she had no way of reaching it. The vastness of what lay out there, just out of sight, suddenly made her world seem small.

She questioned herself constantly. What if she'd imagined Lee Gon and Shin-jae? What if they were the result of a traumatic blow to the head, rather than real memories? She dreaded the thought that Lee Gon might have forgotten her in his timeline. Even worse, that he might have given up searching and moved on. But in the end it was the flute that tipped her over the edge.

"How can I be the only one that hears it? And why?" She'd never been one to sit by and wait for clues. This was a mystery that needed to be solved, even if she felt woefully unqualified to investigate it.

"Where can I find books on parallel universe theory?" She asked a librarian the following weekend. It was the same library Gon had visited in another lifetime. She may not have been a science buff, but she thought it was at least worth a try.

The librarian gave Tae-eul a once-over, as if to say "you don't look the type", and then led her to a secluded section of the building where dusty physics books were piled high. Tae-eul gathered a large selection on the theory of time travel, cosmology and the multiverse, and then spent the day trawling through pages that were dense with jargon and tiny footnotes. "If only you could see me now, Gon, you wouldn't think of teasing me about rabbits on the moon."

After several weeks of research, Tae-eul started noticing the same explanations and theories popping up in different resources. It was frustrating to feel like she was reaching the bottom of the barrel. She now understood multiverse theory to a whole new level, but she wasn't getting any closer to bridging the gap between what science could prove and what she'd personally experienced.

"Why don't more time travellers write books?" she mused on her way home one evening. Another tiring day of research was over. She was eating a steamed bun, but almost dropped it when music burst through the air.

She hadn't heard the flute all day, but now it was louder than ever. She looked around. There was still no sign of anyone. She kept walking a few paces, and the sound seemed to retreat. She backtracked. Louder again. It was coming from a definite point this time, she was sure of it.

She continued backing up until it reached a crescendo, and then stopped in her tracks. There in front of her was a small antique shop. It looked mostly filled with junk, but something about it made the hairs on the back of her neck stand up. It was the same feeling she'd experienced in the bamboo forest, when Gon had first shown her the gateway. Something about it beckoned her forward.

She pushed open the grimy door and a bell chimed above, but the noise was barely noticeable to Tae-eul. Her whole mind was filled with the sound of the flute. Tae-eul pushed past the side tables, plush chairs and worn rugs, until she reached a cabinet stacked with curiosities.

Staring at the small wooden object in front of her, she realised she'd finally found the answer she'd been searching for all those months. Lee Gon's voice came back to her with perfect clarity. "Beautiful equations are always simple." She hadn't understood how maths could be beautiful at the time, but now that the complete solution was before her she thought her heart might burst with excitement.

"Can I help you?" a man asked from behind the counter. The voice jerked Tae-eul back to the present.

"Uh - yes," Tae-eul stuttered. She told herself to stay calm. She realised that if she celebrated too soon, she might feel even worse if it all came to nothing. She needed to pull herself together.

Ten minutes later Tae-eul marched out of the store clutching a wooden flute. She headed straight for the bamboo forest.