ee Ji-hwan (Cha Tae-hyun) receives photographs in the mail that reminds him of how he met two girls, Shim Su-in (Son Ye-jin) and Kim Gyeong-heui (Lee Eun-ju), one summer five years ago. He shares his memories with his friend Cheol-hyeon (Park Young-woo). He recalls initially being in love with Su-in, but she rejects him. Throughout the summer, the three remain friends. Ji-hwan and Gyeong-heui develop feelings for each other but are reluctant to admit it. Ji-hwan recalls the complex relationship between the three to Cheol-hyeon.

Overview from themoviedb.org