The Man From the Basement Is Locked in My Room [Kaizen Scan]

Jeong Gae-seok, living an ordinary office life. But that ordinary life was the result of his obsessive efforts.
His struggle to maintain this normalcy was on the verge of collapse due to the thumping noises coming from the basement apartment every night.
Making a mistake at work because he couldn’t sleep, pushed to a state of extreme sensitivity.
For Gae-seok, whose only goal was to perform “normalcy,” those thumping sounds were nothing less than a ticking time bomb threatening his life.
Finally, Gae-seok went down to the basement apartment to complain about the noise.
As his protests grew heated, the door opened, revealing a strangely familiar-acting, mysterious man.
Ignoring Gae-seok’s complaints, the man went up to Gae-seok’s apartment and tied his own ankle to the window frame.
A man who locked himself in Gae-seok’s room.
What was his purpose, and what was the cause of the thumping sounds echoing from the basement?