Thud! Thud! Thud! The noise between rooms 401 and 501 has started! Eun-soo, an aspiring writer who is no different from a jobless person, begins to observe the owner of the house upstairs, questioning the noise between the floors that tormented her while searching for a subject for a contest. Ho-kyung, the man upstairs, also began to notice the existence of Eun-soo, and eventually, Eun Soo faces a shocking truth.