Director Jo Sung-hee first wrote the script while studying at the Korean Academy of Film Arts and the script went through several rewrites before it was finalized in its current form. This is Jo's commercial debut; he previously directed the arthouse flick End of Animal and the short film Don't Step Out of the House.
Kim Sun-yi, an elderly woman in her sixties living in the US, receives a phone call about the sale of her old family home back in South Korea. Returning to her homeland, she's met by granddaughter Eun-joo, and they drive to the house in the country and stay the night. Sun-yi recalls how 47 years ago when she was a teenage girl in 1965, she moved from Seoul along with her widowed mother and sister Sun-ja to a remote valley to undergo a period of convalescence after suffering problems with her lungs. The Kims lived in genteel poverty at the mercy of their arrogant and foppish landlord, Ji-tae, son of the business partner of Sun-yi's late father. Because of her delicate health, the beautiful yet introverted Sun-yi lives an isolated life in the country home, without any friends her age.
One night, Sun-yi glimpses a shadow in the outhouse; the next day, she discovers a feral boy of about 19 crouching in their yard. The boy's body temperature is 46 degrees Celsius, his blood type unidentifiable, and he can neither read nor speak. Even though he behaves like a wild beast, Sun-yi's kindhearted mother adopts him and names him Chul-soo, assuming he's one of more than 60,000 children orphaned in the Korean War.
At first Sun-yi considers him a nuisance, but eventually has fun taming him according to a dog-training manual. She teaches him how to wait patiently before a meal, how to wear clothes, how to speak, how to write and other human behavior so that he could one day live like a normal man. Chul-soo demonstrates unswerving loyalty and superhuman brawn, thus inspiring the envy of Ji-tae, who lusts after Sun-yi.
As Sun-yi attempts to "civilize" the beast, the two eventually become very close. Sun-yi opens her heart to Chul-soo, and he in turn falls in love with her, the only person to ever show him affection. But their relationship is fraught with difficulties, as Ji-tae begins to cause trouble. Feeling threatened, Chul-soo lets loose his bestial instincts, and in their fear the town villagers turn on him. In order to save the life of the boy who risked his life to be by her side, Sun-yi leaves him with a promise: "Wait for me. I’ll come back for you".
Returning to present day, Sun-yi decides to stay the night. In the middle of the night she walks into the shed to find Chul-soo sitting there, still as young as he was 47 years ago. He hands her the note that she wrote. She realizes that he's been there waiting all along. He reads her the book that she asked him to when he can speak, as she sleeps. The next day she wakes up with Chul-soo nowhere in sight. She leaves with her granddaughter. In the car they receive a call from the county asking about the property. Sun-yi tells him that she's not selling the place and hangs up. Chul-soo stares from afar as the car drives away.
A sequence in the ending credits shows Chul-soo building a snowman.Song Joong-ki - Chul-soo
Park Bo-young - young Sun-yi / Eun-joo
Lee Young-lan - Kim Sun-yi
Jang Young-nam - Sun-yi's mother
Yoo Yeon-seok - Ji-tae
Kim Hyang-gi - Sun-ja
Yoo Sung-mok - Professor Kang Tae-shik
Seo Dong-soo - army colonel
Woo Jeong-guk - Mr. Jung
Gu Bon-im - Mrs. Jung
Nam Jung-hee - Dong-seok's grandmother
Ahn Do-gyu - Dong-seok
Shin Bi - Dong-mi
Lee Jun-hyeok - policeman
Oh Yeong-seok - policeman
Lee Sung-ju - Sun-yi's son
Jang Seo-yi - Sun-yi's daughter-in-law
Jo Jae-yun - Sun-yi's grandson
The film's music video featured John Park's single "철부지" ("Childlike").
"My Prince," the song that Sun-yi sings in the film, was released as a digital single and included in the soundtrack. It was composed by music director Shim Hyun-jung with lyrics by director Jo Sung-hee.
After premiering at number one in the South Korean box office with more than 100,000 admissions, A Werewolf Boy broke the 1 million mark after five days, 2 million after nine days, and 3.6 million in twelve days. Not only were these numbers remarkably high for November, considered a slow season for moviegoing in Korea, but it was also a rare feat for its melodrama genre.
The film also has the distinction of setting a new box office record for "suneung day," the date on which high school seniors take their College Scholastic Ability Test. Each year large numbers of students book tickets for films in the evening after the exam has finished, but A Werewolf Boy's one-day score of 341,475 tickets on November 8 outpaced the totals of any film in previous years.
On November 15, its 4.12 million admissions surpassed Architecture 101 to become the most successful Korean melodrama of all time. Ticket sales reached 5 million on November 18, 6 million on November 26, then 7 million on December 16, making it the third highest Korean top grosser of 2012, behind The Thieves and Masquerade, and also the fourth best selling film of the year overall.
The film also became a sleeper hit when it was released in Taiwan on December 28, 2012, grossing NT$4 million (US$138,000) at the Taipei box office after 17 days on release.
The film also made its premiere in the Philippines on September 18, 2013 as part of the Korean Movie Festival 2013.
After director Jo Sung-hee revealed during one of the film's Q&A sessions that they had shot an alternate ending, due to popular demand, the movie was re-released on December 6, 2012 with that ending. The alternate finale involves Park Bo-young's Sun-yi, and among the deleted scenes are moments from Ji-tae's (Yoo Yeon-seok) childhood as well as more focus on the neighborhood in which the plot unfolds.
A novelization was published on October 31, 2012, to coincide with the movie's opening day.