Jeong Soo-na (Go Eun-ah) is a 17-year-old high school student who lives with her uncle, Se-jin (Jeong Yu-seok), and grandmother (Jeong Yeong-sook) in a big mansion. Her bullied best friend, Lee Han-jeong (Lee Da-in) is subjected to a cruel prank that humiliates her at a public place that she decides to commit suicide by jumping on the railway. She returns home, but becomes a hikikomori (a reclusive lifestyle) until her bully arrives to apologize, only to find Han-jeong's dead body littered with splattered blood and vermin. The news drives Soo-na over the edge, while Han-jeong's bully commits suicide by jumping from the rooftop because of her trauma.
Soo-na slowly becomes a hikikomori, as identified by her uncle's fiancée, Choi Yoon-mi (Chae Min-seo), a psychology student. This is compounded when Se-jin's old flame, Han Song-i (Lee Yeon-soo) meets with Se-jin, only to be followed and beaten by Soo-na's grandmother as the latter angrily refuses to allow her to meet with Soo-na. During a dinner, Soo-na, in a trance, recounts a story to Yoon-mi that implies that Se-jin is actually her father, with Song-i her mother, and that the two were involved in a teacher-student romance until Se-jin abandoned her, with Song-i afterward marrying a man who abuses her everyday. Yoon-mi is furious when Se-jin avoids her questions and states that she will not talk with him unless he brings the topic again.
Soo-na's condition becomes even worse, with vermin starting to appear on her body. When the housemaid enters Soo-na's room while she is out, she is knocked out and fitted inside a barrel with the corpse of the Jeong's pet dog. Soo-na's grandmother attempts to reach Soo-na's room by a ladder but Soo-na kicks it, breaking her grandmother's back. That night, her grandmother heads to Soo-na's room but is stopped by Soo-na and a figure behind her, the latter of whom makes Soo-na's grandmother falls off of the stairs to her death.
Se-jin leaves a voice message to yoon-mi, telling that he needs her, in a desperate attempt to talk with Soo-na again. Song-i is later found dead on the railway. Upon hearing this, Se-jin talks to Soo-na in front of her bed room door, stating that he left Song-i not because he stopped loving her, but because she was already married by the time he was discharged from the army. When Se-jin falls asleep later, Soo-na observes her mother's spirit fondling Se-jin. Meanwhile, Yoon-mi searches Song-i's house and learns that Se-jin and Song-i have a second daughter, Kim Mi-jeong (Song Min-jeong) who lives with her mother. She quickly goes to Se-jin's mansion, where Se-jin is about to commit suicide until he sees what appears to be Soo-na. He follows her but is knocked out. When he wakes up, he discovers Yoon-mi lying motionless in Soo-na's bed. He sees Soo-na about to jump and tries to convince her, only to realize that she is not Soo-na; she is actually Mi-jeong. He lets her go and Mi-jeong then dies from impalement.
Soo-na is transferred to a mental facility where she admits to letting Mi-jeong plot against their father and grandmother, since Mi-jeong, who had to live in an abusive family life with her stepfather, is jealous of Soo-na's happy life with their father. Se-jin reads Mi-jeong's hikikomori blog and learns that she was the one who had committed all the horrible things that happened in their house, not Soo-na. The film ends with Se-jin repeatedly piercing a knife to his table as vermin covers him, indicating that he is becoming a hikikomori.Go Eun-ah ... Jeong Soo-na
Jeong Yu-seok ... Jeong Se-jin
Chae Min-seo ... Choi Yoon-mi
Jeong Yeong-sook ... Grandmother
Lim Dae-ho ... Gwan Ri-in
Lee Da-in ... Lee Ha-jeong
Lee Yeon-soo ... Han Song-i
Lee Eun ... Lee Eun-hee
Jeong Da-hye ... Teacher
Song Min-jeong ... Kim Mi-jeong
The set of the mansion cost US$300,000 to build, and took a month to construct.
Loner was released in South Korea on 18 September 2008, and on its opening weekend was ranked seventh at the box office with 45,710 admissions. As of 5 October, the film had received a total of 74,753 admissions, and as of 12 October had grossed a total of US$412,300.
James Mudge of Beyond Hollywood described Loner as "one of the better and more thoughtful films of its type for some time", and said, "The film features some skilfully sustained tension and a palpable sense of dread, as well as an overbearing sense that things are unlikely to work out well for any of the characters, even for those who manage to make it through to the end credits. Whilst there are a few death scenes and some good use of special effects, the film is generally quite low key, and is arguably all the better for it, as this ensures that the psychological chills remain at the fore."