The Day a Pig Fell into a Well depicts the criss-crossing desires and lives of four characters in diverse circumstances. Novelist Hyosup (Kim Yui-seong) has yet to publish a single book worth mentioning. After learning that his manuscript is merely gathering dust at his friend's publishing company, Hyosup gets into a fight with a reviewer and spends the night in jail. Suffering from an inferiority complex and paranoia at being treated as a third-rate novelist, Hyosup falls passionately in love with a married woman named Bokyung (Lee Eung-kyeong). Her husband Dongwoo (Park Jin-seong), who suffers from a severe case of OCD, goes to Jinju on business but cannot shake his distrust of Bokyung. Meanwhile, Minjae (Cho Eun-suk), a reasonably vain and deluded woman who works behind the ticket counter at a movie theatre, dreams of becoming Hyosup's wife. She derives happiness from proofreading his manuscripts, but he has no time for Minjae due to his absorption in the adulterous affair with Bokyung. Believing that she can get her husband to take her back whenever she wants, Bokyung decides to escape her married life with Hyosup. She packs her things and waits for him to turn up at the appointed hour, but he fails to keep his promise. She goes to his attic room, only to find the door locked and no sign of life coming from behind it. Sitting in a cat atonic state inside Hyosup's room is Minsu, who has murdered Minjae and her beloved Hyosup after she rejected his suit. With no inkling of what has really taken place, Bokyung returns to her home. The next morning, Bokyung opens the newspaper. She then lays it down along the swath of sunlight on her balcony, and walks upon it.