The central twist: Choi Sung-jae, Sun-woo's warm brother-in-law and a police officer, is not what he seems. He is possessed by an evil spirit. He was responsible for the hit-and-run that injured Sun-woo's sister Eun-hee, and he murdered Soon-ae because she witnessed it.
Soon-ae's true grudge: For most of the series, Soon-ae believes she is trapped on earth because she died a virgin. The finale shows that her real unfinished business was finding out how she died and who killed her. Once the truth is out and detectives tell her father about the murder, she walks into a bright light and leaves for good.
The romance: Sun-woo and Bong-sun become an official couple, and Sun-woo's feelings end up anchored on the real Bong-sun rather than the ghost's personality. Soon-ae's own late love is bittersweet, a clear case of love lost to timing.
Hidden meaning: Looking back, the virgin-ghost comedy is a misdirection. The show quietly ties together grief, justice and closure, and the lustful premise turns out to have been a cover for a story about unresolved death.