This article aims to analyze the concepts and techniques used in the composition of an award-winning original soundtrack, One Summer’s Day. This music piece is composed by Joe Hisaishi, a Japanese composer, featuring in the internationally-acclaimed Japanese film animation, Spirited Away (2001). The composition using the technique of thematic transformation and runs for 5.40 minutes in length. The analysis shows that the composition builds its unity by using the technique of the fourth interval. Melody and harmony are constructed by many concepts, such as the quartal chord, the suspended chord, the extended chord, the 4-note group of a minor pentatonic scale, and Lydian mode. Aside from the left-hand and right-hand melody, many concepts, such as upper structure chord, blocked chord, arpeggio, and the bass line have also been incorporated to enrich the sound dimension.