One day, while working at a convenience store, Lee Ji-hwan receives a package containing a photo album and a letter. The letter is from Shim Su-in, whom he had not seen in the five years since they last met. As he leafs through the album, Ji-hwan is reminded of the summer he spent with Su-in and Kim Gyeong-heui.
Ji-hwan recalls how he first met Su-in at a bookstore and was immediately smitten with her. However, Su-in rejects him, saying that she's not interested in dating anyone. Despite this, the two become close friends and Ji-hwan spends most of his summer with her, accompanying her to various places and helping her with her work at a photography studio.
Ji-hwan also becomes friends with Gyeong-heui, a girl who works at the same convenience store as him. The three of them spend a lot of time together, exploring the city and having fun. Over time, Ji-hwan begins to have feelings for Gyeong-heui, but is unsure if she feels the same way.
Throughout the movie, there are several flashbacks to that summer, showing the wonderful memories that the three created together. However, as the summer comes to an end, Ji-hwan, Su-in, and Gyeong-heui drift apart. Ji-hwan goes to study abroad, Su-in moves away, and Gyeong-heui stays in Seoul to work.
Back in the present day, Ji-hwan decides to track down Su-in and reconnect with her. However, he finds out that she has married and moved to the United States. He then decides to visit Gyeong-heui, hoping to rekindle their friendship or possibly something more.
When Ji-hwan arrives at Gyeong-heui's apartment, she initially doesn't recognize him and tells him to leave. However, after seeing the photo album that he brought with him, she finally remembers him. They spend the night talking about their past and reminiscing about the fun times they had together.
The next day, Gyeong-heui tells Ji-hwan that she has always loved him and that she regrets not telling him earlier. They then exchange a heartfelt kiss and it's implied that they start a romantic relationship. The movie ends with Ji-hwan receiving another package from Su-in, this time containing a photo that was missing from the album. Ji-hwan smiles as he realizes that even though things didn't work out with Su-in, he still has a chance at happiness with Gyeong-heui.