The Man Locked up in My Room

Gae-Seok leads an ordinary office life, but that life is the product of his obsessive efforts... His efforts to live a normal life are threatened by the thumping noises coming from downstairs every night. He's sleep-deprived and hyper-sensitive, making mistakes at work. To Gae-Seok, whose sole goal is to portray "ordinariness," the thumping noises are a life-threatening time bomb. Eventually, Gae-Seok descends to his semi-basement room to complain about the noise. As his protests intensify, he opens the door to find a mysterious man with a strangely friendly demeanor. He ignores Gae-Seok's protests, goes up to Gae-Seok's house, and ties his ankles to the windowsill...

The man who locked himself in Gae-Seok's room. What is his motive? And what was the source of the thumping noises echoing from the semi-basement?

(Source: Lezhin, translated)