The film is a melodrama about two lovers in the summer of 1969. In the film, Lee plays college student Yoon Suk-young who finds love while participating in a rural home-stay program over the summer with his classmates. The film's love interest, a librarian named Suh Jung-in, is played by actress Su Ae.

"Once in a Summer" is set in the rustic countryside. Yoon Suk-young (played by Lee Byung-hun) is a rich college student who goes with his friends to help out in the village in order to escape his demanding father. He chances upon Suh Jung-in (Su Ae) who works in the village library her father built. Suh had been abandoned by her parents who defected to North Korea and she is left to fend for herself. Despite the different ideological backgrounds that Yoon and Suh come from, Yoon soon falls for Suh and eventually persuades her to return to the city with him.

The budding romance is cut short when they both unintentionally get involved in a student protest march. Both were thrown into jail and questioned. The officers discover that Yuh's parents are communists and after being interrogated on their relationship, Yoon denies knowing Suh, sealing the fate of both lovers from that moment on. Yoon manages to get out of jail through his father's connection. Suh, however, is thrown into jail.

Yoon waits for her release but their reunion is shortlived. Yoon takes her to the train station with the intention to start their lives afresh. But Yuh finds an opportunity to shake Yoon off and disappears out of his sight forever. The ill-fated lovers, whose love began in the 10 days Yoon was in Yuh's village, never find a happy ending. By the time Yoon finds Suh many years later, he finds only a box of things that she has left behind.

While the show takes its time concocting the first meeting of the lovers, the story of the romance between the two leads rushes headlong into a wall and leaves little time for the romance to develop. The starting ends up being a bit long-winded and a test of patience and does little but show the cat and mouse games between the boy and girl before he lands his catch.

The movie wakes up from its stupor when the male lead decides to stay back to express his feelings to his co-star and to take her back to Seoul and you hope that the character development will finally begin. However, it ends up hitting the first of many plateaus instead.

While it is interesting for the romance to be set in an era of political upheaval between North and South Korea, the characters themselves seem dissociated from the setting. For a couple whose love affair is abruptly aborted by the political situation, it is, at best, a heart-wrenching era in which their romance is set in. It was as if the director, halfway through filming, decided to up the notch on the fate of the lovers by setting the story in a politically-charged time in the country's history. The director fails to draw the characters into the situation he sets them in and hence he ends up with two characters that don't seem to know the complications their love presents.

The movie hits more plateaus than peaks and washes down as a so-so movie from Korea. We have seen much better movies since the Korean wave hit South east Asia. This one will go down as a forgettable movie, at best.