Happy End is about Choi Bora (Jeon Do-yeon), a successful career woman who becomes involved with her ex-lover, Kim Il-beom (Joo Jin-mo). Bora's home life is a snore: she's mother to an infant child and her husband, Seo Min-ki (Choi Min-sik) has lost his job, leaving Bora as the family's sole breadwinner. It's unclear if Bora is with Kim just for the sex or for the passion, both of which Seo seems incapable of giving. But it seems the jobless Seo hasn't been just wandering around parks and reading romance novels as first thought; he knows something is going on, and he's collecting evidence.

Seo has been emasculated by his inability to find a job and director Jung hammers this point home with a brief montage showing Seo grocery shopping, cooking, and doing the laundry. These are all very feminine jobs, particularly in very patriarchal South Korea. Most interesting is that Seo seems content to live with the cheating Bora, very much aware of his own shortcomings, which leaves him willing to be wronged.

Bora is unable to stop going back to Kim even though she seems physically and emotionally damaged by their continued affair. Kim has realized that he is hooked on her, and is very aware of his jealously-driven actions toward her and her family. Without each other, they have no passion in their lives, and so they must keep going back to each other.

Although Happy End ends rather unhappily, the film is not altogether downbeat. Director Jung Ji-woo has taken the role of observer, using mostly handheld cameras to capture the events in the lives of his 3 main subjects.

The film is sexually explicit, and there is one scene of brutal violence.

Part of the film was shot at a branch of ECC (English Center for Children). In the movie, Bora was a director of an English "Hak won" or cram school. The Seong Nam branch of ECC was used because it was an actual English tutoring school. In one scene, Bora is showing a mother and her child one of the classes taught by an American instructor. The mother in the scene was the real school's manager of foreign teachers, and the English instructor was an actual American teacher hired as an extra. His name is James Reynolds, and he works now in America teaching high school Social Studies. He said he was paid $150 for his role.