Apt

Se-jin, who lives in an old apartment in a Seoul suburb, amuses herself by observing the windows of the apartments on the other side. One day, she notices that the lights of some of these apartments are turned off at the very same time.

Apt is a 2006 horror thriller movie, starring Ko So-young, Kang Seong-jin, Yûko Fueki. It holds an IMDb rating of 5.7/10 from 1,179 votes and runs 1h 30m.